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title: FBG Interrogator Guide: Methods, Specs, Sizing | DuyuSense
url: https://www.duyusense.com/en/guides/fbg-interrogator
---

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# What is an FBG interrogator and how do you choose one?

Short answer

An FBG interrogator is the optoelectronic instrument that illuminates a fiber, reads the reflected Bragg wavelength of every FBG sensor on it and converts each wavelength shift into strain, temperature, acceleration or pressure. Choosing one means matching wavelength range, channel count, sampling rate and repeatability to the application. DuyuSense develops a patented FBG interrogation system for real-time, high-speed measurement in harsh environments.

Last updated: 2026-09-30 · Prepared by: Dr. Serhat Boynukalın, DuyuSense

## What does an FBG interrogator actually do?

An FBG interrogator sends light into one or more optical fibers and measures the peak wavelength reflected by each Fiber Bragg Grating (FBG). Every grating reflects a narrow band around its own Bragg wavelength, so the instrument sees a comb of peaks, one per sensor.

When a sensor is strained or heated, its peak moves. Near 1550 nm the shift is typically about **1.2 pm per microstrain** and about **10 pm per °C**. The interrogator tracks these shifts with picometre-level stability and applies calibration factors to output engineering units.

In practice the interrogator is the heart of the system: the sensors are passive glass, while the interrogator sets the achievable speed, resolution, sensor count and data interface.

- Input: light returning from passive FBG sensors (no electrical power at the sensor)
- Core task: peak detection and wavelength tracking for every grating
- Output: time-stamped strain, temperature, acceleration or pressure values over Ethernet or industrial protocols

## Which interrogation methods are used, and how do they differ?

Three architectures dominate: swept (tunable) laser, broadband source with spectrometer, and edge-filter or ratiometric designs. Each makes a different trade-off between speed, resolution, optical budget and cost.

A **swept laser** interrogator scans a narrow-linewidth laser across the band and records reflected power against time, which maps directly to wavelength. It offers high optical power and a wide dynamic range, which suits long fibers and many sensors per channel. A **broadband source plus spectrometer** illuminates all gratings at once and captures the spectrum on a detector array, with no moving parts and good speed. **Edge-filter** methods convert wavelength shift into an intensity ratio, enabling very high sampling rates for a small number of sensors.

- Swept laser: strong optical budget, many sensors, typically suited to static and moderate dynamic measurement
- Spectrometer based: compact, robust, simultaneous capture of all peaks
- Edge filter: very fast, low sensor count, well suited to vibration and acoustic work

## Which specifications should you compare when buying an FBG interrogator?

Compare six figures side by side: wavelength range, number of optical channels, sensors per channel, sampling rate, wavelength resolution and repeatability, and whether the unit is designed for static or dynamic measurement. Together they define how many points you can monitor and how fast.

Wavelength range and grating spacing set the sensor count per channel: a range of about 40 nm (typically around 1510 to 1590 nm) shared by gratings that each need a few nanometres of working window gives roughly 10 to 30 sensors per fiber. Repeatability, usually quoted in picometres, is more meaningful than display resolution because it is what survives temperature drift and long-term operation.

- Wavelength range (nm): available spectral room for sensors
- Channels: independent fibers read in parallel or by an optical switch
- Sensors per channel: depends on range and each sensor's working window
- Sampling rate: from a few Hz for structural monitoring up to kHz for vibration
- Resolution and repeatability (pm): determines the smallest reliable strain or temperature change
- Static vs dynamic: long-term drift stability versus bandwidth for fast events

## How is temperature compensated in FBG strain measurement?

Temperature compensation uses a second, strain-free FBG exposed to the same temperature, and subtracts its wavelength shift from the strain sensor's shift. Because a bare grating responds to both strain and temperature, this separation is essential for any long-term strain reading.

The compensation grating is housed loosely in a tube or package so it cannot pick up mechanical load. Engineers also correct for the thermal expansion of the host structure, since steel or concrete expanding with temperature produces real strain that the sensor measures. Other approaches include athermal packaging and dual-grating designs with different sensitivities, solved as a two-equation system.

- Reference FBG: free-floating grating reading temperature only
- Host correction: account for the structure's thermal expansion coefficient
- Dual-parameter sensors: two gratings with different strain and temperature coefficients

## What types of packaged FBG sensors are available?

FBG sensors are packaged for five common measurands: strain, temperature, acceleration, pressure and tilt. The package transfers the physical quantity into a controlled strain on the grating while protecting the fiber.

Strain sensors are weldable, bonded or embedded in concrete and composites. Temperature sensors isolate the grating from load. Accelerometers attach the grating to a spring-mass element, pressure sensors use a diaphragm, and tilt sensors use a pendulum or inclined mass. Several types can share one fiber, which simplifies cabling in large projects.

- Strain: weldable, surface-bonded, embedded or composite-integrated
- Temperature: strain-free housing, often used as the compensation reference
- Accelerometer: spring-mass design for vibration and seismic response
- Pressure: diaphragm-based for fluids, wells and pipelines
- Tilt and displacement: pendulum or lever mechanisms for rotation and movement

## How do you size an FBG monitoring system?

Size an FBG system by starting from the measurement points, then working out sensors per fiber, channel count and sampling rate. The interrogator is chosen last, once these numbers are known.

First list every point and measurand, including one temperature reference per strain zone. Group points into fiber lines by location, then check that each line's sensors fit in the wavelength range with enough spacing for their expected full-scale shift. The required sampling rate follows from the fastest phenomenon to be captured, typically at least several times its highest frequency. Finally, add optical loss from connectors and cable length and keep spare channels for future expansion.

- Count points and measurands, including temperature references
- Allocate a wavelength window per sensor from its full-scale range
- Derive channels from sensors per fiber and routing
- Set sampling rate from the fastest event of interest
- Check optical budget and plan spare capacity

## What does DuyuSense offer in FBG interrogation and sensor design?

DuyuSense develops a **patented FBG interrogation system** designed for real-time, high-speed measurement in harsh environments, together with proprietary FBG sensors engineered for sensitivity and durability. The company is based at ITU Teknopark in Istanbul.

Beyond the interrogator, DuyuSense offers custom sensor design, prototyping from concept to field-tested prototype, system integration, consultancy and training. Where long-distance vibration coverage is needed, the team also develops Distributed Acoustic Sensing (DAS) and can combine both technologies. DuyuSense is open to pilots and B2B project partnerships through contact@duyusense.com.

- Patented FBG interrogation: real-time, high-speed, harsh environments
- Proprietary FBG sensors focused on sensitivity and durability
- Custom sensor design and field-tested prototyping
- System integration, consultancy and training support

**FBG sensor vs electrical (foil) strain gauge**

| Criterion | FBG sensor | Electrical strain gauge |
|---|---|---|
| Signal type | Optical wavelength shift, immune to electromagnetic interference | Resistance change, sensitive to EMI and cable noise |
| Sensors per cable | Many sensors multiplexed on one fiber | Typically one sensor per cable (multi-wire) |
| Cable length | Kilometres with low signal loss | Limited by lead resistance and noise |
| Power at sensor | None, passive glass | Excitation voltage required |
| Hazardous or high-voltage areas | Well suited, no electrical current in the field | Needs special protection and isolation |
| Long-term drift | Low, absolute wavelength encoding | Zero drift possible, sensitive to moisture and bonding |
| Initial cost | Higher interrogator cost, lower per-point cost at scale | Low cost per point for small systems |

## Frequently asked questions

### What is an FBG interrogator?

An FBG interrogator is the instrument that sends light into an optical fiber and measures the wavelength reflected by each Fiber Bragg Grating sensor. It converts each wavelength shift into strain, temperature, acceleration or pressure using calibration factors, and delivers time-stamped data to monitoring software. The sensors themselves are passive, so the interrogator defines speed, resolution and capacity.

### How many FBG sensors can one interrogator channel read?

It depends on the wavelength range and on how much spectral room each sensor needs. With a typical range of about 40 nm and a few nanometres per sensor, one channel commonly reads around 10 to 30 sensors. Multi-channel interrogators multiply this, so a single unit can handle from tens to hundreds of measurement points.

### What is the difference between a swept laser and a spectrometer interrogator?

A swept laser interrogator scans a tunable laser across the band and records reflections over time, giving strong optical power and suiting long fibers with many sensors. A spectrometer interrogator uses a broadband source and a detector array to capture all peaks at once, with no moving parts. The choice depends on speed, sensor count and optical budget.

### What sampling rate do I need for FBG monitoring?

The sampling rate should be several times the highest frequency of the phenomenon being measured. Slow structural or thermal monitoring typically needs a few Hz or less, while vibration, modal analysis and impact detection often need hundreds of Hz to several kHz. Choosing a faster rate than needed increases data volume without improving accuracy.

### How do you separate strain and temperature with FBG sensors?

The standard method places a strain-free reference FBG next to the strain sensor at the same temperature and subtracts its wavelength shift. Engineers also correct for the thermal expansion of the host structure. Alternatives include athermal packaging and dual-grating sensors with different sensitivities, from which strain and temperature are solved as two equations.

### Is an FBG sensor better than an electrical strain gauge?

FBG sensors are preferable where electromagnetic interference, long cable runs, hazardous areas or many measurement points are involved, because one passive fiber can carry many sensors over kilometres. Electrical strain gauges remain cost-effective for small laboratory setups with short cables. For large or harsh-environment projects, FBG usually lowers cabling effort and long-term drift.

### How long do FBG sensors last in harsh environments?

Properly packaged FBG sensors are designed for many years of service, because the grating is written into glass and has no electronics at the measurement point. Service life depends mostly on the package, adhesive or weld, and cable protection. Choosing gratings and packaging suited to the temperature, humidity and mechanical loads of the site is key.

### Does DuyuSense design custom FBG sensors and interrogation systems?

Yes. DuyuSense develops a patented FBG interrogation system for real-time, high-speed measurement in harsh environments and proprietary FBG sensors focused on sensitivity and durability. It also offers custom sensor design, prototyping up to field-tested prototypes, system integration and consultancy. Pilot and partnership enquiries can be sent to contact@duyusense.com.

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