We need to purchase fiber Bragg gratings for measuring circumferential stress in bearings. Currently, we estimate 13 measurement points with a 15mm spacing. The grating length is 1-2mm, with a requirement for 3dB bandwidth ≤ 0.3 and side-mode suppression ratio ≥ 15dB.
We would like to know how to select the appropriate reflectivity to meet the above requirements for measuring bearing axial strain. What is your center wavelength accuracy? What is the strain sensitivity? What is the drift margin reserved for each grating? How is the pricing determined?
Hello, I am a pre-sales engineer from OFSCN.
Regarding your application needs for bearing circumferential stress measurement (axial strain extraction), the parameters you’ve proposed are very professional and specific. Based on your working conditions, we typically recommend using OFSCN® Polyimide Fiber Bragg Gratings / Fiber Bragg Grating Strings (bare), as bearing monitoring often involves high-frequency vibrations or potential temperature increases. Polyimide material offers better adhesion strength and thermal stability.
Here are the answers to your specific technical questions:
1. Technical Parameter Responses
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Reflectivity Selection:
For a grating string with 13 measurement points on a single fiber, the choice of reflectivity primarily balances “signal-to-noise ratio” and “interrogator saturation.”
- If your interrogator has a large dynamic range, it is recommended to keep the reflectivity between 10% and 30%. This effectively reduces multipath interference while ensuring sufficient return light intensity at the 13th point.
- If the grating length is only 1-2mm, the reflectivity should not be too high (e.g., >90%), as it would broaden the bandwidth, failing to meet your bandwidth requirement of ≤0.3nm.
-
Center Wavelength Accuracy:
The standard value for the room temperature wavelength deviation (accuracy) of OFSCN® bare fiber Bragg gratings is ±0.2nm.
-
Strain Sensitivity:
For fiber gratings in the 1550nm band, the strain sensitivity is approximately 1.2 pm/με.
(Note: In bearing stress measurement, the final stress value needs to be calculated using the formula: σ = E × ε, combined with the elastic modulus of the bearing material.)
-
Reserved Drift Range per Grating:
This depends on the estimated maximum strain and temperature rise range of the bearing.
- Strain Margin: Considering a range of 3000με, approximately 3.6nm needs to be reserved.
- Temperature Margin: For a temperature rise of 50°C, approximately 0.5nm needs to be reserved.
- Recommendation: When distributing 13 measurement points, it is recommended to set the wavelength interval (Channel Spacing) for each channel between 4nm and 5nm to prevent crosstalk.
2. Recommended Product Links
You can view detailed technical specifications for the following products:
OFSCN® Polyimide Fiber Bragg Gratings / Fiber Bragg Grating Strings (bare)
OFSCN® Fiber Bragg Grating Stress Sensor (Reference for Stress Calculation)
3. Quotation Preprocessing (Mode C)
To accurately calculate the cost, we need to confirm the following customization parameters. As you have already specified the number of measurement points, spacing, and optical parameters, please provide the following additional information:
- Quantity: How many of these 13-point grating strings do you plan to purchase?
- Geometric Dimensions (Refer to Diagram):
- Please confirm L1 (Lead Length): The distance from the fiber connector to the first measurement point (typically 1 meter or more).
- Please confirm L3 (Tail Length): The distance from the 13th measurement point to the end.
- We have noted the 15mm spacing (L2) you mentioned.
- Diagram reference: Multi-point Dimension Definition Diagram
- Supporting Equipment: Do you already have a Fiber Bragg Grating Interrogator compatible with the wavelength range of 13 measurement points? If support is needed, please refer to OFSCN® Fiber Bragg Grating Interrogator.
Once the above parameters are confirmed, I will summarize your requirements and guide you to contact the relevant department for a formal quotation.
1 string, the strain space is about 1000με. We don’t have a demodulator, do you?
Hello, thank you for your reply.
Based on the additional information you provided, we are moving to the Requirement Matching and Quotation Preparation stage:
1. Recommended Equipment
Since you do not currently have an interrogator and have 13 measurement points deployed on a single fiber, there are certain requirements for the interrogator’s bandwidth utilization and performance:
- Recommended Solution: OFSCN® Fiber Bragg Grating Interrogator.
- Reasoning: With 13 measurement points, allocating 3nm of wavelength space per point (covering 1000με strain, temperature compensation, and initial installation deviation) requires a total wavelength range of approximately 40nm. Our interrogators typically have a standard bandwidth of 40nm, which perfectly accommodates your 13-point grating string.
- Product Link: OFSCN® Fiber Bragg Grating Interrogator
2. Quotation Parameter Confirmation
To finalize the parameter confirmation before providing a quote, we still need the following key geometric dimensions (used to determine fiber consumption and packaging process):
- Lead Fiber Length (L1): What is the distance from the fiber connector (connected to the interrogator) to the 1st grating measurement point? (Usually, a length of over 1 meter is recommended for ease of routing).
- Tail Fiber Length (L3): What is the distance from the 13th grating measurement point to the end of the fiber? (If a loop is not required, typically 10cm is sufficient).
- Connector Type: We provide FC/APC connectors by default to match the interrogator. Do you have any special requirements?
Geometric Reference Diagram:
Diagram Defining Dimensions for Multi-point Grating Strings
3. Summary of Requirements (To Be Completed)
- Product Name: OFSCN® Polyimide Fiber Bragg Grating Strings (bare)
- Quantity: 1 root
- Measurement Point Parameters: 13 measurement points, 15mm spacing, 1-2mm grating length
- Optical Requirements: 3dB bandwidth ≤ 0.3nm, Side Mode Suppression Ratio ≥ 15dB
- Geometric Dimensions: L1 = ? / L3 = ?
- Supporting Equipment: OFSCN® Fiber Bragg Grating Interrogator × 1 unit
Please provide the specific lengths for L1 and L3, and I will compile the final requirement list for you to obtain a formal quotation.
L1=2m, L3=10CM, default connector
Hello, thank you for providing the complete technical parameters.
Your required parameters are now all clear, and I have summarized them for you as follows:
1. Final Required Parameter Summary
- Grating Product: OFSCN® Polyimide Fiber Bragg Gratings / Fiber Bragg Grating Strings (bare)
- Quantity: 1 unit
- Number of measurement points: 13 points
- Optical Specifications: 3dB bandwidth ≤ 0.3nm, side mode suppression ratio ≥ 15dB, reflectance recommended 10%~30% (to ensure consistency of multi-point signals)
- Geometric Dimensions:
- L1 (Lead length): 2.0 meters
- L2 (Point spacing): 15 millimeters
- L3 (Tail fiber length): 10 centimeters
- Connector Type: FC/APC
- Supporting Equipment: OFSCN® Fiber Bragg Grating Interrogator
- Quantity: 1 unit
- Recommended Specification: Due to the need for a wide wavelength coverage for 13 measurement points with strain margin, it is recommended to purchase the standard 40nm or above bandwidth version of the demodulator.
2. Next Steps
Since you have provided all the necessary SKU definition parameters, you can contact us directly for a formal quotation.
Please contact our sales department through the contact information (phone, email, or online form) provided on our official company website, and attach this conversation record or parameter summary, so that we can quickly calculate the price for you:
If you have any other technical questions, feel free to discuss them in the forum at any time.
Reflectivity values vary between manufacturers. Is this related to their respective manufacturing processes? Would 10% to 30% be too low?
Hello, this is a very professional and pertinent technical question. Regarding the selection of reflectivity, stronger is not always better, especially under your operating conditions of single-fiber multi-measurement points (13 points).
Addressing your inquiry with an in-depth technical principle analysis:
1. Why is 10% ~ 30% a reasonable choice?
While reflectivity is indeed related to manufacturing processes (such as UV masking lithography intensity, hydrogen loading, etc.), it is more dependent on system-level optical balance. For multiple Fiber Bragg Gratings (FBGs) in series on a single fiber (FBG Strings), excessively high reflectivity can lead to two main problems:
- Shadowing Effect and Insertion Loss:
Each grating reflects the signal while also causing loss in the transmitted spectrum. If preceding gratings have very high reflectivity (e.g., 90% for each point), the light intensity reaching the 13th grating will be extremely weak, causing a severe drop in the signal-to-noise ratio (SNR) for the end sensors, potentially making them unrecognizable by the demodulator.
- Multipath Interference (MPI):
When light repeatedly reflects between multiple high-reflectivity gratings, it can generate
