A method for coupling an optical device with increased return loss and an optical receiving device

Abstract

This invention provides an optical device coupling method and optical receiving device that can increase return loss. The method includes: S1, setting up an online return loss monitoring system; S2, obtaining a reference coupling position; S3, determining whether the return loss value corresponding to the reference coupling position meets a preset return loss index; S4, if the return loss value does not meet the index, laterally moving the fiber array and selecting lateral offset candidate points on both sides of the reference coupling position; S5, at the lateral offset candidate points, verifying whether the coupled optical power of each channel of the fiber array meets the index; if all channels meet the index, the coupling positioning is completed; S6, if the coupled optical power / return loss of some channels does not meet the requirements, first axially moving the fiber array and then laterally moving the fiber array, and then determining whether the coupled optical power / return loss meets the index. This invention utilizes offset to disrupt the reversibility of the optical path, causing the back-reflected light to deviate from the numerical aperture angle of the fiber and unable to return along the original path, thus increasing the return loss through a physical mechanism without modifying the FA itself or adding additional optical components.

Description

Technical Field

[0001] This invention belongs to the field of optical communication technology, specifically relating to an optical device coupling method and an optical receiving device that can increase return loss. Background Technology

[0002] In high-speed optical modules (such as 800G and 1.6T), the optical receiver typically uses a fiber array (FA) with lenses to couple the optical signal to the photodiode (PD) photosensitive surface. As single-wavelength rates increase to 200G and above, the PD photosensitive surface size shrinks to 8μm–10μm, and the PD is usually eutectic-mounted on a TIA chip. To ensure coupling efficiency and yield, a lens (such as an aspherical lens, an FA end-face mount lens, or a PD surface-integrated lens) is usually introduced between the FA and the PD. However, because the lens is fixed relative to the PD photosensitive surface, at typical optical path coupling positions that aim for maximum coupling efficiency, the proportion of reflected light returning to the fiber from the PD surface increases, leading to degraded return loss. Measured values ​​are typically only 20dB–25dB, which is insufficient to meet the communication system's requirement of ≥27dB return loss.

[0003] Currently, the existing technical solutions for optimizing the return loss in the lens coupling optical path mainly include: (1) adding an isolator, which is costly and bulky and not conducive to high-density integration; (2) applying special coatings to the lens and PD, which has a narrow process window and low batch yield; (3) rotating the FA as a whole around the front end of the FA, which requires the coupling equipment to increase the degree of rotation, and the equipment structure is complex and the modification cost is high.

[0004] Therefore, how to effectively increase the return loss of lens-coupled optical receivers without significantly sacrificing coupling efficiency, increasing device complexity, or increasing packaging costs has become an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to provide an optical device coupling method that can increase return loss, and at least solve some of the defects existing in the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for coupling optical devices that can increase return loss, used to couple an optical signal output from an optical fiber array to the photosensitive surface of a photodiode, includes the following steps:

[0008] S1. Provide a receiving device for the optical signal to be coupled and build an online return loss monitoring system. The receiving device for the optical signal to be coupled includes an optical fiber array and a photodiode. The online return loss monitoring system synchronously collects the input optical power and the back-reflected optical power in real time during the coupling process and calculates the real-time return loss value.

[0009] S2. Perform optical path pre-alignment on the optical receiving device to be coupled, and obtain the reference coupling position where the fiber array achieves maximum coupling efficiency relative to the photodiode.

[0010] S3. Read the retracement value corresponding to the benchmark coupling position and determine whether the preset retracement index is met.

[0011] S4. If the return loss value at the reference coupling position does not meet the standard, the fiber array is moved step by step along the lateral displacement axis perpendicular to the optical path axis, and the corresponding coupling optical power curve and return loss curve are obtained synchronously by scanning. At least one set of lateral offset candidate points are selected on both sides of the reference coupling position, and the lateral offset candidate points meet the requirements of the return loss value and the coupling optical power attenuation does not exceed the preset upper limit.

[0012] S5. Move the fiber array to the selected lateral offset candidate point and check whether the coupling optical power of each channel of the fiber array meets the power threshold. If all channels meet the standard, the coupling positioning is completed and the solidification and encapsulation are performed. If the coupling optical power / return loss of any channel does not meet the standard, proceed to S6.

[0013] S6. Move the fiber array back to the reference coupling position, offset it by 6~10μm in the positive X-axis direction, and then offset it by 5~10μm in the positive or negative Y-axis direction; if the channel coupling optical power / return loss does not meet the standard, the coupling is judged to be poor.

[0014] Furthermore, in S1, the online return loss monitoring system includes a light source, a three-port fiber optic circulator, an optical power meter, and a control terminal; the output of the light source is connected to the first port of the fiber optic circulator, the second port of the fiber optic circulator is connected to the optical receiving device to be coupled, the third port of the fiber optic circulator is connected to the optical power meter, and the control terminal is electrically connected to the light source and the optical power meter respectively; the real-time return loss value RL = -10log (P) is calculated. back / P in ), where P in For the input optical power, P back This represents the power of the back-reflected light.

[0015] Furthermore, the photodiode is a back-illuminated integrated lens PD, with the lens and the PD photosensitive surface being eccentrically set, the eccentric distance being 6-10 μm, and the lens curvature radius being 95-125 μm; the fiber array is an inclined end face fiber array with an end face inclination angle of 42.5°.

[0016] Furthermore, S2 also includes a pre-calibration process: visually identifying the pitch angle of the fiber array, adjusting the clamping posture of the fiber array until the pitch angle is less than 1° before performing optical path pre-alignment.

[0017] Furthermore, the preset retracement index is a retracement value ≤ -32dB.

[0018] Furthermore, in S4, the step size of the lateral displacement axis is 0.2 to 1 μm, and the lateral offset is controlled within 5 to 10 μm. The rule for selecting candidate points for lateral offset is: select the points where the return loss curve reaches the preset return loss index for the first time, and the decrease in coupled optical power compared with the reference coupling position is not greater than 15% as candidate points for lateral offset.

[0019] Furthermore, select one lateral offset candidate point on each side of the reference coupling position, compare the absolute values ​​of the offsets of the two lateral offset candidate points relative to the reference coupling position, and select the point with the smaller absolute value of the offset as the final lateral offset candidate point.

[0020] In addition, the present invention also provides an optical receiving device, which is fabricated using the above-mentioned optical device coupling method, including a circuit board, a transimpedance amplifier, a photodiode, and an optical fiber array; the transimpedance amplifier is mounted on the circuit board, and the photodiode is eutectic mounted on the transimpedance amplifier; the optical fiber array is laterally offset by a preset offset along the vertical optical path axis relative to the reference coupling position corresponding to the maximum coupling efficiency, and the preset offset ensures that the return loss of the optical receiving device meets a preset return loss requirement, and the coupled optical power received by the photodiode meets a preset power requirement.

[0021] Furthermore, the photodiode integrates a lens, and the lens and the photosensitive surface of the photodiode are arranged with an off-center distance of 6 to 10 μm and a lens curvature radius of 95 to 125 μm; the end face of the fiber array is a beveled end face with an angle of 42.5°.

[0022] Furthermore, the preset offset value is 5 to 10 μm.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) This invention makes the fiber array laterally offset off-axis by a preset offset relative to the reference coupling position corresponding to the relative maximum coupling efficiency. The offset destroys the reversibility of the optical path, causing the back-reflected light to deviate from the numerical aperture angle of the fiber and unable to return along the original path. This physically increases the return loss. This method is simple and easy to implement, and does not require modification of the FA itself or the addition of additional optical components.

[0025] (2) This invention achieves real-time monitoring and closed-loop feedback of return loss during coupling by designing an online return loss monitoring system, eliminating the need for offline testing and greatly improving process efficiency.

[0026] (3) The present invention adopts a dual threshold determination mechanism, which simultaneously limits the return loss threshold and the coupling power threshold, and bidirectionally constrains the lateral offset point of the fiber array to ensure that the coupling efficiency of the device is not excessively lost due to offset while meeting the system return loss requirements, thus effectively controlling the yield.

[0027] The present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the optical receiving device structure of the back-illuminated integrated lens PD in an embodiment of the present invention;

[0029] Figure 2 This is a schematic diagram of the coupling direction of the optical receiving device structure in an embodiment of the present invention;

[0030] Figure 3 This is a schematic diagram of the optical receiving device structure using discrete lenses in an embodiment of the present invention;

[0031] Figure 4 This is a schematic diagram of the optical receiving device structure using the FA end-face lens in an embodiment of the present invention;

[0032] Figure 5 This is a hardware connection diagram of the online return loss monitoring system in an embodiment of the present invention;

[0033] Figure 6 This is a schematic diagram of the optical device coupling method in an embodiment of the present invention;

[0034] Figure 7 This describes the relationship between the measured return loss curve and the coupled optical power curve in an embodiment of the present invention.

[0035] Explanation of reference numerals in the attached diagram: 1. Circuit board; 2. Transimpedance amplifier; 3. Photodiode; 4. Fiber optic array; 5. Lens. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] In the description of this invention, it should be understood that the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0038] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation", "connection", and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, an abutting connection, or an integral connection. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0039] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature; in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0040] like Figures 1 to 6 As shown, this embodiment provides an optical device coupling method that can increase return loss, used to couple the optical signal output from an optical fiber array to the photosensitive surface of a photodiode, including the following steps:

[0041] S1. Provide a receiving device for the optical signal to be coupled and build an online return loss monitoring system. The receiving device for the optical signal to be coupled includes an optical fiber array and a photodiode. The online return loss monitoring system synchronously collects the input optical power and the back-reflected optical power in real time during the coupling process and calculates the real-time return loss value.

[0042] Specifically, such as Figure 5 As shown, the online return loss monitoring system includes a light source, a three-port fiber optic circulator, an optical power meter, and a control terminal. The output of the light source is connected to the first port (port1) of the fiber optic circulator, the second port (port2) of the fiber optic circulator is connected to the optical receiving device to be coupled, the third port (port3) of the fiber optic circulator is connected to the optical power meter, and the control terminal is electrically connected to the light source and the optical power meter respectively. The real-time return loss value RL is calculated as -10log(P). back / P in ), where P in For the input optical power, P backThe back-reflected light power is shown below. The working principle of this online return loss monitoring system is as follows: The optical fiber circulator emitted by the light source enters the optical receiving device to be coupled via port 1→port 2. The back-reflected light generated inside the optical receiving device (from the lens interface, PD surface, etc.) enters the optical power meter via port 2→port 3 of the circulator. The control terminal converts the reflected light power into a return loss value and monitors it in real time, realizing online monitoring and closed-loop feedback of return loss during the coupling process.

[0043] Specifically, the optical path structure with coupled light receiving device in this embodiment is as follows: Figure 1 As shown, the fiber array is a slanted endface fiber array with an endface slant angle of 42.5°; the photodiode is a back-illuminated integrated lens PD, with the lens integrated and mounted on the PD. The lens is offset from the photosensitive surface of the PD by a distance of 6–10 μm, and the lens radius of curvature is 95–125 μm. Alternatively, the optical path structure of the optical receiving device to be coupled can also be such that the lens is discretely positioned between the FA and the PD (e.g., ...). Figure 3 (as shown), or the lens is integrated and mounted on the FA end face (such as...). Figure 4 (As shown).

[0044] S2. Perform optical path pre-alignment on the optical receiving device to be coupled, and obtain the reference coupling position where the fiber array achieves maximum coupling efficiency relative to the photodiode.

[0045] Specifically, in the step of obtaining the reference coupling position, the coupling optical power curves of each channel of the fiber array are scanned along the X / Y / Z directions respectively. When the coupling optical power curves of all channels coincide in space, it indicates that the relative orientation between the fiber array and the photodiode has reached the optimal co-alignment state. At this time, the spatial position of the fiber array is recorded as the reference coupling position.

[0046] The optimized process includes a pre-calibration step before optical path pre-alignment. Specifically, the pitch angle of the fiber array is identified visually (e.g., by taking a picture with a camera), and the clamping posture of the fiber array is adjusted until the pitch angle is less than 1° before optical path pre-alignment is performed. By visually identifying and calibrating the pitch angle of the fiber array to less than 1°, the interference of the beam incident angle introduced by the pitch tilt can be eliminated, making the multi-channel coupling curves coincide and uniform. The return loss value and coupling power data obtained from the reference coupling position have high repeatability. At the same time, since this invention relies on lateral offset to control the return loss, after the pitch angle is controlled within 1°, the pitch tilt angle of the beam incident lens is fixed, and the back reflection is dominated by only the single variable of lateral offset. This ensures the regularity and stability of the subsequent offset scanning and screening of lateral offset candidate points, strictly controls the coupling power attenuation, reduces the performance drift after packaging and curing, and simultaneously improves the detection accuracy and mass production yield of the automated coupling process.

[0047] S3. Read the retracement value corresponding to the benchmark coupling position and determine whether the preset retracement index is met.

[0048] Specifically, when reading the reference coupling position, the return loss values ​​of each channel of the fiber array are denoted as RL1, RL2, ..., RL. n n is the number of channels, and the maximum retracement value among all channels is taken (denoted as RL). max To determine whether the preset pullback index is met, taking four channels (CH1 to CH4) as an example, the pullback values ​​of the four channels are denoted as RL1, RL2, RL3, and RL4, respectively. In this embodiment, the preset pullback index is designed as pullback value ≤ -32dB. Therefore, the maximum pullback value RL among the four channel pullback values ​​is determined. max Is it ≤-32dB? If RL max If the value is ≤-32dB, then coupling and positioning are complete, and solidification and encapsulation can proceed directly.

[0049] S4. If the return loss value at the reference coupling position does not meet the standard, the fiber array is moved step by step along the lateral displacement axis (i.e., the Y-axis direction, the horizontal direction) perpendicular to the optical path axis, and the corresponding coupling optical power curve and return loss curve are obtained synchronously by scanning. At least one set of lateral offset candidate points are selected on both sides of the reference coupling position. The lateral offset candidate points meet the requirements of the return loss value and the coupling optical power attenuation does not exceed the preset upper limit.

[0050] Specifically, the optical coupling power curve and synchronous return loss curve are scanned with a step size of 0.2–1 μm. The return loss curves of each channel (such as RL1–RL4) and RL are acquired in real time through an online return loss monitoring system. max The corresponding pullback curve records all instances where RL is lost. max Meet the preset pullback indicator (such as RL) max From the Y-axis positions of ≤-32dB, select the points where the return loss curve first reaches the preset return loss index and the decrease in coupled optical power compared with the reference coupling position is no more than 15% as candidate points for lateral offset.

[0051] Furthermore, select one lateral offset candidate point on each side of the reference coupling position. The reference coupling position is denoted as Y1, and the two lateral offset candidate points are denoted as Y2 and Y3 respectively. Record the lateral offset ΔY1=|Y2-Y1| and ΔY2=|Y3-Y1|, where the lateral offset ΔY is controlled within 5~10μm. If ΔY1≤ΔY2, then Y2 is selected as the final lateral offset candidate point. If ΔY1>ΔY2, then Y3 is selected as the final lateral offset candidate point.

[0052] S5. Move the fiber array to the selected lateral offset candidate point and check whether the coupling optical power of each channel of the fiber array meets the power threshold. If all channels meet the standard, the coupling positioning is completed and the solidification and encapsulation are performed. If the coupling optical power / return loss of any channel does not meet the standard, proceed to S6.

[0053] Specifically, the power threshold is set according to the specific product specifications, and is usually 85% to 95% of the maximum coupled optical power value (i.e., the coupled optical power value at the reference coupling position).

[0054] S6. Move the fiber array back to the reference coupling position, offset it by 6~10μm in the positive X-axis direction, and then offset it by 5~10μm in the positive or negative Y-axis direction; if the channel coupling optical power / return loss does not meet the standard, the coupling is judged to be poor.

[0055] Taking a four-channel optical path structure as an example, Figure 7 The relationship between the measured return loss curves and the coupled optical power curves of the four channels is shown. As can be seen from the scanning curves, the return loss is worst at the maximum power center position. As the fiber array gradually shifts off-axis along the transverse Y-axis, the back-reflected light is effectively suppressed, and the return loss rapidly deteriorates to below -30dB; simultaneously, the coupled optical power attenuates gradually. By selecting the point with the smaller offset from the return loss compliance boundary on both sides of the maximum power center position, the coupled optical power loss can be controlled within a reasonable range while ensuring that the return loss meets the specifications, achieving a bidirectional balance between return loss and coupling efficiency. This verifies the feasibility of the off-axis offset reflection suppression optical path mechanism and the dual-threshold optimization process of this invention.

[0056] The optical receiver device fabricated using the above-described optical device coupling method includes a circuit board 1, a transimpedance amplifier 2, a photodiode 3, and an optical fiber array 4. The transimpedance amplifier 2 is mounted on the circuit board 1, and the photodiode 3 is eutectic mounted on the transimpedance amplifier 2. The optical fiber array 4 is laterally offset by a preset offset along the vertical optical path axis relative to the reference coupling position corresponding to the maximum coupling efficiency. The preset offset ensures that the return loss of the optical receiver device meets a preset return loss requirement, and that the coupled optical power received by the photodiode 3 meets a preset power requirement.

[0057] Optionally, the photodiode 3 integrates a lens 5, and the lens 5 is positioned with the photosensitive surface of the photodiode 3, with an eccentric distance of 6 to 10 μm and a radius of curvature of 95 to 125 μm; the end face of the fiber array 4 is a beveled end face with an angle of 42.5°.

[0058] Specifically, the preset offset ensures that the return loss of the optical receiving device is ≥32dB (i.e., the return loss value is ≤-32dB), while ensuring that the coupled optical power value received by the PD is ≥ the preset target value (i.e., 85%), thus achieving the optimal balance between return loss and coupling efficiency.

[0059] This invention achieves increased return loss by offsetting the lens and photosensitive surface of the photodetector to a compatible array (FA) structure and actively translating the FA along the Y-axis. This offset disrupts optical path reversibility, causing the back-reflected light to deviate from the numerical aperture angle of the fiber and prevent it from returning along its original path. This is a simple and easy-to-implement design that requires no modification to the FA itself. Furthermore, it employs two criteria: "return loss ≤ -32dB" and "coupled optical power ≥ preset target value," ensuring that the coupling efficiency of the optical receiver does not suffer excessive loss due to offset while meeting system return loss requirements, effectively controlling yield. When the offset fails to meet the power / return loss requirements, recoupling to the maximum value is achieved by moving the FA 5-10µm along the positive X-axis and then translating it along the Y-axis, thus doubly guaranteeing coupling yield.

[0060] The above examples are merely illustrative of the present invention and do not constitute a limitation on the scope of protection of the present invention. All designs that are the same as or similar to the present invention are within the scope of protection of the present invention.

Claims

1. A method for coupling optical devices that can increase return loss, used to couple an optical signal output from an optical fiber array to the photosensitive surface of a photodiode, characterized in that, The steps include the following: S1. Provide a receiving device for the optical signal to be coupled and build an online return loss monitoring system. The receiving device for the optical signal to be coupled includes an optical fiber array and a photodiode. The online return loss monitoring system synchronously collects the input optical power and the back-reflected optical power in real time during the coupling process and calculates the real-time return loss value. S2. Perform optical path pre-alignment on the optical receiving device to be coupled, and obtain the reference coupling position where the fiber array achieves maximum coupling efficiency relative to the photodiode. S3. Read the retracement value corresponding to the benchmark coupling position and determine whether the preset retracement index is met. S4. If the return loss value at the reference coupling position does not meet the standard, the fiber array is moved step by step along the lateral displacement axis perpendicular to the optical path axis, and the corresponding coupling optical power curve and return loss curve are obtained synchronously by scanning. At least one set of lateral offset candidate points are selected on both sides of the reference coupling position, and the lateral offset candidate points meet the requirements of the return loss value and the coupling optical power attenuation does not exceed the preset upper limit. S5. Move the fiber array to the selected lateral offset candidate point and check whether the coupled optical power of each channel of the fiber array meets the power threshold. If all channels meet the standard, the coupling positioning is completed and the solidification and encapsulation are performed. If the channel coupling optical power / return loss is below the standard, then run S6; S6. Move the fiber array back to the reference coupling position, offset it by 6~10μm in the positive X-axis direction, and then offset it by 5~10μm in the positive or negative Y-axis direction. If the channel coupling optical power / return loss is below standard, the coupling is considered poor.

2. The optical device coupling method as described in claim 1, characterized in that, In step S1, the online return loss monitoring system includes a light source, a three-port fiber optic circulator, an optical power meter, and a control terminal. The output of the light source is connected to the first port of the fiber optic circulator, the second port of the fiber optic circulator is connected to the optical receiver to be coupled, the third port of the fiber optic circulator is connected to the optical power meter, and the control terminal is electrically connected to the light source and the optical power meter respectively. The real-time return loss value RL is calculated as -10log(P). back / P in ), where P in For the input optical power, P back This represents the power of the back-reflected light.

3. The optical device coupling method as described in claim 1, characterized in that, The photodiode is a back-illuminated integrated lens PD, with the lens and the PD photosensitive surface being eccentrically set with an eccentric distance of 6-10 μm and a lens curvature radius of 95-125 μm; the fiber array is an inclined end face fiber array with an end face inclination angle of 42.5°.

4. The optical device coupling method as described in claim 1, characterized in that, The S2 also includes a pre-calibration process: visually identifying the pitch angle of the fiber array, adjusting the clamping posture of the fiber array until the pitch angle is less than 1° before performing optical path pre-alignment.

5. The optical device coupling method as described in claim 1, characterized in that, The preset retracement index is a retracement value ≤ -32dB.

6. The optical device coupling method as described in claim 1, characterized in that, In S4, the step size of the lateral displacement axis is 0.2 to 1 μm, and the lateral offset is controlled between 5 and 10 μm. The rule for screening candidate points for lateral offset is: select the point where the return loss curve reaches the preset return loss index for the first time, and the ratio of the decrease in coupled optical power compared with the reference coupling position is not greater than 15% as the candidate point for lateral offset.

7. The optical device coupling method as described in claim 6, characterized in that, Select one lateral offset candidate point on each side of the reference coupling position, compare the absolute values ​​of the offsets of the two lateral offset candidate points relative to the reference coupling position, and select the point with the smaller absolute value of the offset as the final lateral offset candidate point.

8. An optical receiving device, characterized in that, The optical device is fabricated using the optical device coupling method described in any one of claims 1-7, comprising a circuit board, a transimpedance amplifier, a photodiode, and an optical fiber array; the transimpedance amplifier is mounted on the circuit board, and the photodiode is eutectic mounted on the transimpedance amplifier; the optical fiber array is laterally offset by a preset offset along the vertical optical path axis relative to the reference coupling position corresponding to the maximum coupling efficiency, the preset offset ensuring that the return loss of the optical receiving device meets a preset return loss requirement, and that the coupled optical power received by the photodiode meets a preset power requirement.

9. The optical receiving device as described in claim 8, characterized in that, The photodiode integrates a lens, and the lens and the photosensitive surface of the photodiode are positioned with an off-center distance of 6 to 10 μm and a lens curvature radius of 95 to 125 μm; the end face of the fiber array is a beveled end face with an angle of 42.5°.

10. The optical receiving device as claimed in claim 8, characterized in that, The preset offset value is 5 to 10 μm.

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