Choosing the right Double Clad Thulium Doped Fiber is an important step in developing a stable and efficient 2μm fiber laser or amplifier. Core diameter, cladding size, numerical aperture (NA), and pump absorption at 793nm all influence fiber performance. In this guide, we compare four fiber configurations from 10/130 to 25/400, explain their key specifications, and discuss how to match them with different laser applications. Based on our experience at Box Optronics, we also highlight the practical considerations engineers should check before selecting a thulium-doped fiber.
Contents
- How Does Double Clad Thulium Doped Fiber Work?
- What Factors Matter When Choosing Thulium Doped Fiber?
- How Do the Four Fiber Models Compare?
- How to Match Fiber Specifications with Laser Applications?
- What Should Be Checked Before Integrating the Fiber?
- How Does Box Optronics Support Fiber Selection?
- Frequently Asked Questions
1. How Does Double Clad Thulium Doped Fiber Work?
Double clad thulium-doped fiber is an active optical fiber designed primarily for fiber lasers and amplifiers operating around the 2μm wavelength region. It contains a thulium-doped core, an inner pump cladding, and an outer low-refractive-index coating structure.
Unlike conventional single-clad fiber, its double-clad design allows pump light to travel through the larger inner cladding while the laser signal is guided by the doped core. This arrangement makes it possible to couple high-power multimode pump light into the fiber while maintaining controlled signal propagation.
For many 2μm laser systems, a 793nm laser diode is used as the pump source. Thulium ions absorb the pump energy and generate laser emission in the approximately 2μm region. Through the cross-relaxation process between thulium ions, one absorbed pump photon can contribute to the excitation of two upper laser-level ions under suitable conditions, improving energy conversion efficiency.
At Box Optronics, we focus on fiber configurations designed for this operating principle. Our Double Clad Thulium Doped Fiber series covers an operating wavelength range of 1600–2100nm, with different core and cladding dimensions for different laser designs.
2. What Factors Matter When Choosing Thulium Doped Fiber?
In our experience, selecting a fiber based only on its core diameter or nominal operating wavelength is not enough. Several optical and mechanical parameters need to be considered together.
2.1 Core Diameter and Core Numerical Aperture
The core diameter and numerical aperture (NA) influence how the laser signal propagates through the fiber. A smaller core, such as 10μm, is commonly considered for single-mode laser and amplifier designs where beam quality is important.
Larger core configurations, such as 25μm, provide a different mode area and can be more suitable for laser architectures requiring higher power handling. However, a larger core does not automatically guarantee better output performance. The actual result depends on the fiber NA, cavity design, bending conditions, and mode control.
2.2 Cladding Diameter and Numerical Aperture
The inner cladding carries the pump light, so its diameter and NA directly affect pump coupling compatibility. Our available models include 130μm, 250μm, and 400μm cladding diameters, all with a cladding NA of at least 0.46.
A 130μm cladding configuration is more compact and may simplify integration into smaller laser assemblies. A 250μm or 400μm cladding provides a larger pump-guiding area, which may be useful in systems with different pump delivery and power requirements.
2.3 Cladding Absorption at 793nm
Cladding absorption is one of the most important parameters when evaluating a 793nm-pumped thulium fiber. It indicates how effectively the pump light is absorbed per unit fiber length.
For example, the BTDF-25/250 model has a specified cladding absorption of 4.8±0.2dB/m, while the BTDF-15/250 model has 1.0±0.2dB/m. These differences can influence the active fiber length required for a particular pump absorption target.
However, higher absorption does not automatically mean higher laser efficiency. Pump coupling efficiency, residual pump power, fiber background loss, signal gain, and cavity configuration must also be considered.
2.4 Mechanical Strength and Coating Dimensions
Mechanical properties matter during fiber handling, splicing, winding, and long-term operation. All four models in our current specification table have a mechanical strength of at least 100kpsi.
The cladding uses an octagonal shape, while the outer cladding material is low-refractive-index resin. The coating diameter varies from 245μm to 550μm depending on the model. These dimensions should be checked against the selected fiber stripper, fusion splicer, and packaging structure.
3. How Do the Four Fiber Models Compare?
Box Optronics offers four standard configurations in this Double Clad Thulium Doped Fiber series: BTDF-10/130, BTDF-15/250, BTDF-25/250, and BTDF-25/400.
All four models share the same specified operating wavelength range and cladding NA, but their core sizes, cladding dimensions, and pump absorption values are different.
3.1 Optical and Geometrical Specifications
| Parameter | BTDF-10/130 | BTDF-15/250 | BTDF-25/250 | BTDF-25/400 |
|---|---|---|---|---|
| Operating Wavelength | 1600–2100nm | 1600–2100nm | 1600–2100nm | 1600–2100nm |
| Core Diameter | 10.0±1.0μm | 15.0±1.0μm | 25.0±1.0μm | 25.0±2.0μm |
| Core NA | 0.14±0.01 | 0.10±0.02 | 0.09±0.01 | 0.10±0.02 |
| Cladding Diameter | 130.0±2.0μm | 250.0±5.0μm | 250.0±5.0μm | 400.0±5.0μm |
| Cladding NA | ≥0.46 | ≥0.46 | ≥0.46 | ≥0.46 |
| Cladding Absorption @793nm | 3.2±0.2dB/m | 1.0±0.2dB/m | 4.8±0.2dB/m | 2.4±0.2dB/m |
| Cladding Attenuation @860nm | ≤20.0dB/km | ≤20.0dB/km | ≤20.0dB/km | ≤20.0dB/km |
| Cladding Shape | Octagon | Octagon | Octagon | Octagon |
| Coating Diameter | 245.0±10.0μm | 395.0±10.0μm | 395.0±10.0μm | 550.0±10.0μm |
| Mechanical Strength | ≥100kpsi | ≥100kpsi | ≥100kpsi | ≥100kpsi |
| Outer Cladding Material | Low Refractive Index Resin | Low Refractive Index Resin | Low Refractive Index Resin | Low Refractive Index Resin |
| Matrix Material | Quartz | Quartz | Quartz | Quartz |
3.2 Which Model Should You Consider?
| Fiber Model | Main Specification | Selection Consideration |
|---|---|---|
| BTDF-10/130 | 10μm core / 130μm cladding | Consider for compact, single-mode 2μm laser and amplifier designs. |
| BTDF-15/250 | 15μm core / 250μm cladding | Consider when a 250μm pump cladding and relatively lower specified pump absorption are required. |
| BTDF-25/250 | 25μm core / 250μm cladding | Consider for designs requiring a larger core and higher specified pump absorption. |
| BTDF-25/400 | 25μm core / 400μm cladding | Consider for designs requiring a larger pump cladding and a 25μm core. |
These are preliminary selection considerations rather than fixed application limits. The final choice should be verified against the required output power, beam quality, pump diode specifications, fiber length, and splicing conditions.
4. How to Match Fiber Specifications with Laser Applications?
The same thulium-doped fiber may perform differently in different laser architectures. We recommend starting with the intended application and working backward to determine the required fiber structure.
4.1 Eye-Safer 2μm Fiber Lasers
Thulium-doped fiber is widely used in laser systems operating around 2μm. Depending on the system design, these lasers can support continuous-wave or pulsed operation. Core diameter, NA, and pump absorption should be evaluated together to achieve the required output characteristics.
The 10/130 configuration is worth considering for compact single-mode designs, while larger-core configurations may be evaluated for other power and mode-management requirements.
4.2 Fiber Amplifiers
In a 2μm fiber amplifier, the active fiber must provide sufficient gain without introducing excessive background loss or unwanted nonlinear effects. Pump absorption and active fiber length are particularly important.
For amplifier development, we suggest checking the signal wavelength, input signal power, target output power, pump configuration, and required gain before finalizing the fiber model.
4.3 Medical and Surgical Laser Systems
Thulium-doped fiber lasers operating around 2μm are used in various medical and surgical laser platforms. Their suitability depends on the required wavelength, pulse characteristics, output power, and optical delivery system.
For medical equipment development, fiber selection should be coordinated with the complete laser system design, including beam delivery, thermal management, and applicable medical safety requirements.
4.4 Plastic Processing and Industrial Applications
In plastic processing, laser wavelength and material absorption characteristics influence how energy is delivered to the workpiece. A 2μm laser source can be considered for specific polymer processing tasks, but the actual result depends on material composition, thickness, processing speed, and beam parameters.
Thulium-doped fiber can also be used in radar-related and other specialized optical systems where a suitable 2μm laser source is required.
5. What Should Be Checked Before Integrating the Fiber?
Even when the nominal specifications match the design, integration issues can affect the final laser performance. We recommend checking the following points during prototype development.
- Pump compatibility: Confirm that the pump diode wavelength, fiber NA, and pump combiner specifications are compatible with the selected fiber.
- Active fiber length: Calculate the expected pump absorption based on the specified absorption coefficient, then verify the actual pump distribution experimentally.
- Splicing compatibility: Check core alignment, mode-field mismatch, and splice loss when connecting the active fiber to passive matching fibers.
- Bending conditions: Follow the recommended bend radius and handling requirements to avoid additional optical loss or mechanical damage.
- Thermal management: Evaluate heat generation and temperature stability under the intended pump power and operating conditions.
- Output verification: Measure output power, slope efficiency, residual pump power, and beam quality under the actual laser configuration.
One point worth emphasizing is that cladding absorption alone cannot predict the final laser slope efficiency. A complete evaluation requires both fiber-level measurements and system-level testing.
6. How Does Box Optronics Support Fiber Selection?
At Box Optronics, we understand that choosing an active fiber is only one part of developing a complete optical system. Engineers often need to consider pump sources, fiber coupling, passive matching fibers, and other optical components at the same time.
Shenzhen Box Optronics Technology Co., Ltd. is based in Shenzhen, China. We provide fiber optic modules, fiber-coupled lasers, laser components, and customized optical product solutions, mainly serving optical fiber communication and optical fiber sensing applications.
With our R&D, production, and sales capabilities, together with experienced product R&D engineers, we work with customers to evaluate technical requirements and identify suitable optical product configurations.
For Double Clad Thulium Doped Fiber projects, we can discuss the following information before recommending a model:
- Target operating wavelength within the 1600–2100nm range.
- Required core and cladding diameters.
- 793nm pump source specifications and absorption requirements.
- Target output power and laser or amplifier configuration.
- Splicing requirements and compatibility with passive matching fibers.
Rather than selecting a fiber simply because it has a larger core or higher absorption value, we prefer to evaluate the complete application requirements. This helps reduce unnecessary changes during prototype integration.
To learn more about our optical products, visit Box Optronics or explore our Thulium Doped Fiber product range.
7. Frequently Asked Questions
Q1: Why Is 793nm Commonly Used to Pump Double Clad Thulium Doped Fiber?
A: Thulium-doped fiber has a suitable absorption band around 793nm. Pumping at this wavelength can take advantage of the cross-relaxation process between thulium ions, allowing efficient conversion of pump energy into emission around 2μm under suitable material and operating conditions.
Q2: Does Higher Cladding Absorption Always Mean Better Laser Efficiency?
A: No. Higher cladding absorption means that pump power is absorbed more strongly per unit fiber length under the specified measurement conditions. However, overall laser efficiency also depends on pump coupling, quantum efficiency, background loss, signal propagation, and cavity design.
Q3: What Is the Main Difference Between 10/130 and 25/400 Thulium Doped Fiber?
A: The main differences are their core and cladding dimensions. The 10/130 model has a 10μm core and 130μm cladding, while the 25/400 model has a 25μm core and 400μm cladding. These structures offer different mode areas and pump coupling characteristics, so the appropriate choice depends on the laser architecture and target performance.
Q4: How Do I Estimate the Required Length of Thulium Doped Fiber?
A: Start with the cladding absorption value at 793nm and the required pump absorption target. Under an idealized uniform-loss model, the remaining pump power can be estimated using the absorption coefficient and fiber length. In practice, actual absorption also depends on pump launch conditions, mode distribution, and fiber characteristics. Experimental verification is recommended.
Q5: Can Double Clad Thulium Doped Fiber Be Used in Both Continuous-Wave and Pulsed Lasers?
A: Yes. Double-clad thulium-doped fibers can be incorporated into both continuous-wave and pulsed 2μm laser systems. However, pulse energy, peak power, repetition rate, nonlinear effects, and thermal conditions must be considered when designing the laser.
Q6: What Information Should I Provide When Requesting a Customized Thulium Doped Fiber Solution?
A: We recommend providing the target wavelength, core and cladding dimensions, pump wavelength, required absorption, output power, and intended application. If you already have a laser design, sharing the pump configuration and passive fiber specifications will also help our engineers evaluate compatibility.
For additional technical information or product selection assistance, contact Box Optronics to discuss your 2μm fiber laser or amplifier requirements.