Optical Fiber FAQs

Optical Fiber, General

A typical refractive index profile of nLIGHT’s single -clad fibers is shown in the figure below. The silica cladding surrounding the core has a refractive index of about 1.45 at 1060 nm. Based on this information, the core refractive index can be estimated from the core numerical aperture of the fiber. The refractive index difference between the core and the cladding can be assumed to be approximately constant between 900 nm and 1600 nm. The polymer coating surrounding the cladding has a higher refractive index preventing any light guidance in the cladding.

A typical refractive index profile of nLIGHT’s double -clad fibers is shown in the figure below. Only the core and inner cladding material are based on glass. The silica cladding surrounding the core has a refractive index of 1.45 at 1060 nm, based on which the core refractive index can be estimated for a given core numerical aperture of the fiber. The refractive index difference between the core and the cladding can be assumed to be approximately constant between 900 nm and 1600 nm. In order to guide light inside the inner cladding, a dual-layer fluoroacrylate polymer coating is utilized. The first coating layer of the polymer coating consists of a low-index fluoroacrylate with a refractive index of about 1.37, whereas the second coating layer is a high index acrylate.

nLIGHT is the first laser fiber manufacturer specifying the core NA directly from the fiber and not the preform refractive index measurements. Unique advantages of realNA for our customers:

✓ Most accurate fiber core NA

✓ Reduced variation in the actual fiber NA

✓ Superior predictability and control of beam quality and fiber performance

✓ Ultimate matching and minimal splice loss between active and passive fibers.

More details can be found here.

nLIGHT’s double-clad fibers are coated by a dual-layer fluoroacrylate polymer. The first coating layer, a low-index fluoroacrylate, is optimized for optical properties and light guidance inside the inner cladding. The second coating layer is a high-index acrylate, which has been optimized for mechanical and environmental protection. Our most recent coating material, which has been introduced to all our standard double-clad fibers during 2013 - 2014, has been particularly developed to sustain higher temperatures and higher power operation. Detailed tests have confirmed that nLIGHT’s double-clad fibers sustain operating temperatures between -40˚C and +120˚C, show no degradation in water immersion / humidity tests and have a very good long-term mechanical reliability (n-value > 22). Please contact an nLIGHT fiber representative for additional information. 

Yes. Please contact an nLIGHT fiber representative for purchasing recoating material suitable for nLIGHT’s fibers. 

Our currently used high-temperature coating for double-clad fibers has been tested and approved for operating between -40˚C and +120˚C. Our currently used coating for single-clad fibers has been tested and approved for operating between -40˚C and +85˚C.

We recommend to store nLIGHT's fibers between +10˚C and +60˚C and within humidity limits of 30% - 70% RH. Protect the fibers from UV and keep the fibers in a dust free environment with the fiber spools in their protective covers.

Passive Fibers

nLIGHT offers different single- and double-clad passive fibers, tailored to match the optical guiding properties of nLIGHT’s Ytterbium-doped fibers. This allows optimal mode coupling between all elements of a fiber laser/amplifier and minimal splice losses. 

We do not offer specially dispersion engineered matching passive fibers for our Erbium doped fibers, as telecom fibers, such as SMF-28, are typically compatible with our Erbium products.

nLIGHT does not separately offer photosensitive fibers since FBGs can be written into all our passive fibers. Many fiber-optic component manufacturers are using nLIGHT’s passive fibers on a standard basis for writing high-quality FBGs.

Ytterbium Doped Fibers

Please contact an nLIGHT fiber representative to receive representative data for the absorption and emission cross section of nLIGHT’s Ytterbium doped fibers. 

The emission cross section is independent of the polarization axis.

nLIGHT’s Ytterbium doped fibers have a fused silica cladding and a core based on aluminosilicate glass.

The optimal length of the fiber depends on the application and ideally should be determined based on simulations taking the exact design into account. An initial estimate can be obtained when assuming a total absorption of 13 dB for the design of a fiber amplifier, and 20 dB for the design of a fiber laser.

An intrinsic feature of Ytterbium doped fibers is the strong wavelength dependent absorption and emission cross section in the vicinity of 976 nm. For this reason the absorption at 976 nm will sensitively depend on the emission wavelength and spectral width of the used pump diodes. As a general rule of thumb, we recommend to multiply the specified absorption at 920 nm by a factor of about 2.5 in order to obtain an estimate for the absorption at 976 nm.

We currently specify the core background loss / intrinsic core loss at 1200 nm for all background sensitive fibers to be below 15 dB/km and below 25 dB/km for all non-background sensitive fibers.

Depending on the fiber type, we recommend the following bending diameters:

Please note, that in practice the optimal bending diameter to obtain best beam quality will also depend on the light launching conditions.

In general, we recommend the heat sink to which the fiber is attached to should be air-cooled, liquid-cooled, or TEC cooled. The heat sink preferably has a groove or grooves for the fiber to reduce the thermal resistance from the fiber to the heat sink. Also splice points should be packaged and thermally managed. The thermal resistance from fiber to heat sink can be further reduced preferably by potting the fiber, but also a soft polymer or a heat paste (customers have also been using UV curable silicone) can be used between the fiber and the heat sink. The maximum operating temperature is +120˚C under non-condensing conditions.

Erbium Doped Fibers

Please contact an nLIGHT fiber representative to receive representative data for the absorption and emission cross section of nLIGHT’s Erbium doped fibers. 

The dispersion parameter of our Erbium doped fibers sensitively depends on the core diameter and core numerical aperture. From simulations, which assume the nominal core diameter and NA, one can expect the dispersion parameter to be in the range of:

*Valid for the wavelength range from 1500 nm – 1600 nm

The effective core area of our Erbium doped fibers depends on the core diameter and core numerical aperture. From simulations, which assume the nominal core diameter and NA, one can expect the effective area of the core to be in the range of:

*Valid for the wavelength range from 1500 nm – 1600 nm

Depending on the fiber geometry, the following nominal nonlinear refractive index can be expected: 

*Valid for the wavelength range from 1500 nm – 1600 nm

Taking the overlap of the fundamental mode with the core into account and depending on the fiber type, one can expect the following Erbium ion densities: 

*Valid for the wavelength range from 1500 nm – 1600 nm

We do not offer specially dispersion engineered matching passive fibers for our Erbium doped fibers.  Standard telecom fibers are typically compatible with our Erbium products.

Please contact an nLIGHT fiber representative to obtain the measured background loss at 1300 nm of your fiber. Please provide the fiber code of your fiber with the inquiry. 

The nominal core diameter and Erbium doping diameter are as follows: 

The spontaneous emission lifetime can be assumed to be about 9 ms for all our Erbium doped fibers.

The fraction of quenched ions (Erbium clustering) is as follows: 

The optimal length of the fiber depends on the application and ideally should be determined based on simulations taking the exact design into account. An initial estimate can be obtained when assuming a total absorption of 70 dB (600 dB) for C-band (L-band) applications. Accordingly, the fiber lengths are: