High Performance Lasers
Optical Fiber, General

What is the refractive index profile of nLIGHT’s single clad fibers?
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 1.4573, which can be assumed to be approximately constant between 900 nm and 1600 nm. Based on this information, the core refractive index can be estimated from the core numerical aperture of the fiber. The polymer coating surrounding the cladding has a higher refractive index preventing any light guidance in the cladding. 


What is the refractive index profile of nLIGHT’s double-clad fibers?
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 can be assumed to have a refractive index of 1.4573 (valid for 900 nm to 1600 nm), based on which the core refractive index can be estimated for a given core numerical aperture of the fiber. 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. 

What is realNA?
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.

What are the properties of the coating material used in nLIGHT’s double-clad fibers?
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. 

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

What are the recommended operation temperatures for nLIGHT’s double-clad optical fibers?
Our currently used high-temperature coating for double-clad fibers has been tested and approved for operating between -40˚C and +120˚C. 

What are the recommended storage conditions for nLIGHT’s optical fibers?
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

Does nLIGHT offer suitable passive fibers for its active 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.
 
Does nLIGHT offer photosensitive passive fibers?
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

What is the absorption and emission cross section of nLIGHT’s 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. 

Does the emission cross section of nLIGHT’s Ytterbium doped polarization maintaining fibers depend on the polarization axis?
The emission cross section is independent of the polarization axis.

What is the core and cladding material of nLIGHT’s Ytterbium doped fibers?
nLIGHT’s Ytterbium doped fibers have a fused silica cladding and a core based on aluminosilicate glass.

What is the recommended fiber length for nLIGHT’s Ytterbium doped fibers?
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.

Where do I find information on the absorption at 976 nm for nLIGHT’s Ytterbium doped fibers?
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.

What is the core background loss in nLIGHT’s Ytterbium doped fibers?
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.

What is the minimum bending radius of nLIGHT’s Ytterbium doped fibers?
Depending on the fiber type, we recommend the following bending diameters:

Fiber Type Min. Coiling Diameter for Good LP01 Transmission [cm] Mechanically Safe Coiling Diameter [cm]
Ybxxx-10/125DC(-PM) 5 3.75
Ybxxx-12/125DC(-PM) 4 3.75
Ybxxx-20/125DC(-PM) 9 3.75
Ybxxx-20/400DC(-PM) 12 12
Ybxxx-25/250DC(-PM) 8 7.5
Ybxxx-30/250DC(-PM) 9 7.5

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

What is the recommended cooling method for nLIGHT’s Ytterbium doped fibers?
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

What is the absorption and emission cross section of nLIGHT’s 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. 

What is the dispersion of nLIGHT’s standard 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:

Fiber Geometry Nominal Dispersion [ps/(nm*km)]
Erxxx-4/125 -12 ... -18
Erxxx-8/125 10 ... 16
*Valid for the wavelength range from 1500 nm – 1600 nm

What is the effective core area of nLIGHT’s Erbium doped fibers?
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:

Fiber Geometry Nominal Effective Area [(m²)]
Erxxx-4/125 26 ... 32
Erxxx-8/125 60 ... 70
*Valid for the wavelength range from 1500 nm – 1600 nm

What is the nonlinear coefficient of nLIGHT’s Erbium doped fibers?
Depending on the fiber geometry, the following nominal nonlinear refractive index can be expected: 

Fiber Geometry Nominal Nonlinear Refractive Index n2 [(cm²/W)]
Erxxx-4/125 2.0 10.0-16 ... 2.2  10.0-16
Erxxx-8/125 2.4 • 10.0-16 ... 2.5 • 10.0-16
*Valid for the wavelength range from 1500 nm – 1600 nm

What is the Erbium ion density of nLIGHT’s Erbium doped fibers?
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: 

Fiber Type Erbium Ion Density [(m-3)]
Er16-8/125 6.8 • 10.024
Er30-4/125 2.1 • 10.025
Er40-4/125 3.5 • 10.025
Er80-8/125 3.9 • 10.025
Er110-4/125 8.4 • 10.025
*Valid for the wavelength range from 1500 nm – 1600 nm

Do you offer passive fibers matching to your Erbium doped fibers?
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.

What is the background loss at 1300nm of your Erbium doped fibers?
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. 

What is the core diameter and Erbium doping diameter of nLIGHT’s Erbium doped fibers?
The nominal core diameter and Erbium doping diameter are as follows: 

Fiber Type Nominal Core and Erbium Doping Diameter [(m)]
Erxxx-4/125 3.5
Erxxx-8/125 7.6

What is the spontaneous emission lifetime of nLIGHT’s Erbium doped fibers?
The spontaneous emission lifetime can be assumed to be about 9 ms for all our Erbium doped fibers.

What is the fraction of quenched ions (Erbium clustering) of nLIGHT’s Erbium doped fibers?
The fraction of quenched ions (Erbium clustering) is as follows: 

Fiber Type Fraction of quenched ions
Er30-xxx 4.80%
Er40-xxx 7.0%
Er80-xxx 14.0%
Er110-xxx 16.0%

What fiber length do you recommend to use with your Erbium doped fibers?
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: 

Fiber Type Nominal Absorption at 1530nm [dB/m] C-Band Application Length [(m)] L-Band Application Length [(m)]
Er16-8/125 16 4.5 38
Er30-4/125(HC) 30 2.3 20
Er40-4/125 40 1.8 15
Er80-8/125 80 0.9 7.5
Er110-4/125 110 0.6 7.5
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