High Power Laser Assemblies and Their Optical Cables

When choosing optical cable assemblies for power delivery systems, the power limitations of the three central components of cable assembly needs to be considered: the base material, the input connector, and the mode stripper (if used).

The base material is the first thing to consider, specifically the base material interface. Fiberguide’s high power assemblies are constructed with Fiberguide’s Step Index Multimode fiber, which has a pure fused silica core and fluorine doped cladding. The fused silica has a very high purity, and this makes it capable of handling massive amounts of optical energy.

The difficulty lies in getting the optical energy into the fused silica, and this is dictated by the interface between silica and air that is present at the input connector. Fiberguide makes use of a proprietary laser polishing technique to increase the amount of power that this interface can cope with, but it can still regulate the power handling capability of the assembly as a whole.

The error mode for Continuous Wave (CW) Lasers is thermal, triggered by irregularities at a microscopic scale in the air-silica interface absorbing the energy of the laser. The failure mode can be either a thermal or an atom-level dielectric breakdown for Pulsed Lasers, depending on the characteristics of the laser. Either way, there is a maximum power per unit of area, known as the damage threshold, that is capable of being coupled into the assembly. This is reported in W/cm2 (irradiance) for CW lasers, and in J/cm2 (fluence) for Pulsed Lasers. The difference is due to the fact that Pulsed Lasers function as a series of recurring energy bursts, or pulses.

The Peak Power and Average Power of the laser is determined by the duration of pulses and the rate at which they are repeated. As a joule (J) is the quantity of energy needed to create one watt (W) of power for one second, this unit of measure is employed to eliminate time as a factor to make comparisons easy.

Deciding whether or not a laser will damage a fiber depends on calculating the irradiance or fluence for the laser by dividing the CW Power (also called the Energy per Pulse) by the area of the beam where it comes in contact with the fiber. This value has to be amended to compensate for wavelength in a CW laser, and pulse duration in a Pulsed Laser.

The sizes of the beam and fiber are suitable for the laser if the amended value surpasses the damage threshold. If the amended value is higher than the damage threshold, the beam size and/or the fiber size ought to be increased until the irradiance or fluence is less than the damage threshold. The charts and tables included here display the maximum irradiance and fluence values for a number of fiber core sizes by wavelength for CW lasers, and by wavelength and pulse duration for Pulsed Lasers.

The power-handling capacities for the two other main cable assembly components (i.e. the input connector and the mode stripper) also have to be examined. The failure mode on these is thermal every time, and there are detailed segments on these components within this piece.

Continuous Wave (CW) Lasers

Chart 1 depicts the highest power that can be used for several fiber core sizes at usual CW laser wavelengths. Either the air-silica interface damage threshold, the connector, or the mode stripper will be the limiting factor, depending on the power level and wavelength.

The solid lines in the chart represent the interface limits, whereas the dashed lines depict the connector and mode stripper limits. To show the dependency the damage threshold has on wavelength, Chart 1 depicts power levels that far exceed those that are realistically feasible or currently available for some wavelengths.

Chart 1: CW Power Maximimums (up to 1 kW) for Fiber Core Sizes 100 μm ~ 800 μm with λ: 193 nm - 2100 nm

Pulsed Lasers

When determining the power limits for cable assemblies coupled to Pulsed Lasers, the calculations that are used are dictated by the Pulse Duration (τ). For Pulse Durations exceeding 1 µs (10-6 s), the failure mode is thermal and CW calculations (or charts) are used. In this case, the Laser’s Average Power = Energy Per Pulse (J) x Pulse Frequency (Hz) is used in place of CW power.

When Pulse Durations are less than 10 ps (10-11 s), the failure mode is a 2nd order, non-linear phenomenon, such as Stimulated Brillion Scattering (SBS) or Stimulated Raman Scattering (SRS), which is consistently present in optical fibers and becomes a dominant factor at extremely short pulse durations. In such cases, detailed testing of several beam and/or fiber sizes is the optimal way to conclude what is appropriate.

When pulse durations are between 10 ps and 1 µs, the failure mode leans towards a dielectric breakdown at the atomic level, and the Energy per Pulse and Fiber Size are the main components in deciding power maximums.

To decide whether or not a fiber size is adequate, the Energy per Pulse has to be divided by the area of the beam where it comes into contact with the fiber, and the result of this has to be compared to the air-silica damage threshold. This is straightforward if the characteristics of the laser complement those of the damage threshold, but if this isn’t the case then an additional step is required.

In the event where the wavelength and/or pulse duration of the laser do not match those of the damage threshold (λ: 1064 nm and τ: 1 ns), information is used to decide the correction factor (Table 3). This is followed by the multiplication of the Energy per Pulse by the correction factor to arrive at the Equivalent Energy per Pulse at λ: 1064 nm and τ: 1 ns. These are obtained by scaling wavelength linearly where the shorter wavelengths are more damaging, and by scaling pulse duration by square roots where the shorter pulses are more damaging.

After the Equivalent Energy per Pulse at λ: 1064 nm and τ: 1ns have been found, Table 4 can be used to decide the size of the fiber that can be used. The maximums here are established based on the conjectures stated in Table 1.

The last step for Pulsed Lasers is to check the Average Power using the CW calculations, or chart to account for the duty cycle (where Average Power = Energy per Pulse (J) x Pulse Frequency (Hz)). All possible sizes of fiber from the previous step have to be evaluated to guarantee they pass both criteria. In the end, these are the fiber sizes that are capable of being used with a given Pulsed Laser Source.

Example 1

Given that a 532 nm Nd:YAG Pulsed Laser emits 100 ns pulses at 100 Hz frequency, and the Average Power is 5W, which fiber size can be employed?

First, the Energy per Pulse must be determined based on either the Average Power or Peak Power. Energy per Pulse (J) = Average Power (W) / Pulse Frequency (Hz), or, Peak Power (W) x Pulse Duration (sec). In the current example, the Energy per Pulse = 5 W / 100 Hz = 0.05J or 50 mJ.

Because this laser functions at a different wavelength and pulse duration compared to the damage threshold value (λ: 1064 nm and τ: 1 ns), to calculate the Equivalent Energy per Pulse a correction factor has to be employed.

Table 3 shows that the correction factor for λ: 532 nm and τ: 100 ns is 0.20. With this in mind, the Equivalent Energy per Pulse = Laser’s Energy per Pulse x Correction Factor = 50 mJ x 0.2 = 10 mJ. This results in that at 1064 nm and 1 ns; a 10 mJ pulse has the equivalent amount of energy as a 50 mJ pulse at 532 nm and 100 ns.

Table 4 shows that fiber sizes of 400 µm and higher will be suitable for this laser. To consider variations in systems, Fiberguide recommends testing with core sizes above and below the mathematical limits for preferred performance.

To compare, the Peak Power for this Laser = Energy per Pulse (J) / Pulse Durations (sec) = 0.05 / 100x10-9 = 500,000W or 500kW.

Example 2

Imagine a 1064 nm Q-Switched Pulsed Laser gives off 150 ns pulses at a frequency of 30 kHz, and the Peak Power is 75 kW - which fiber size should be used?

Firstly, one should determine the Energy per Pulse. This can be obtained from the Average Power or Peak Power. Energy per Pulse (J) = Average Power (W) / Pulse Frequency (Hz), or, Peak Power (W) x Pulse Duration (sec). In the current example, the Energy per Pulse = 75 kW x 150 ns = 75,000 x 150 x 10-9 = 0.01125 J or 11.25 mJ.

A correction factor has been used to calculate the Equivalent Energy per Pulse because the laser functions at the same wavelength but for a different pulse duration than the damage threshold value (λ: 1064 nm and τ: 1 ns). Because Table 3 does not show 150ns, the correction factor has to be manually calculated using the following formula:

With this formula, the Equivalent Energy per Pulse = Laser’s Energy per Pulse x Correction Factor = 11.25mJ x 0.08 = 0.9 mJ. Thus, at 1064 nm and 1 ns, a 0.9 mJ pulse has the equivalent amount of energy as a 11.25 mJ pulse at 1064 nm and 150 ns.

Table 4 shows that fiber sizes equal to 100 µm or larger will be suitable for this laser. To make up for variations in systems, and in this case due to the fact that 100 µm is at maximum, Fiberguide recommends testing with core sizes ranging from below to above the mathematical limits for preferred performance.

To put this into perspective, the Average Power for this laser = Energy per Pulse (J) x Pulse Frequency (Hz) = 0.01125 x 30,000 = 338W.

Input Connector

An HP SMA or HP FD-80 Connector is used for the input in Fiberguide’s High Power Cable Assemblies. The two connectors offer a precision machined, epoxy free, cantilevered or air gapped nose design that enables safe dissipation of thermal energy without burning adjacent material.

The connectors’ end faces are laser polished to guarantee a high quality optical finish with high quality to ensure the highest power handling capability. These connectors are developed to cope with substantially higher power than conventional flush-polished SMAs, or connectors with ceramic ferrules.

The key difference between the HP SMA and HP FD-80 connector is that the HP FD-80 is a keyed connector. This characteristic permits repeatable angular positioning during times when the cable assembly is disconnected or reconnected. The HP SMA is less rugged than the HP FD-80, which is also larger, which makes it the perfect option for industrial use. The HP FD-80 is also available with a sapphire insert in the base of the nose.

The connectors’ failure mode is thermal overload due to physical size and construction constraints which occur when the materials (organics) used break down. The connectors’ limits for power handling are independent of fiber size, or laser features (laser type, wavelength, pulse duration, etc.) and strictly act as an upper power limit.

Connector Type Maximum Power
HP SMA 650 W
HP FD-80 750 W

 

Mode Stripped Assemblies

Mode Strippers are designed to remove energy from an optical assembly’s cladding and dispel it as heat. They are typically employed in fiber lasers where individual laser diodes are used to pump a lasing fiber. Eliminating the cladding modes at each separate pump stops the build up of heat in places where managing it is impossible, thus aiding in maintaining the system’s numerical aperture (NA).

Additionally, Mode Strippers are employed in alternative applications where it is advantageous to eliminate the cladding energy in an expected way, close to the source. Examples of this include systems disposed to poor input beams or poor alignment, cable runs with pointed bends, or fusion splices that could lead to hot spots.

In Mode Strippers, as increasing energy is removed, the connector becomes hotter until there is internal thermal failure. In the event this occurs prior to reaching the damage threshold of the air-silica interface - or prior to reaching the power maximum for the input connector - the Mode Stripper is can control the maximum power rating of the assembly.

In a classic system, cladding constitutes 2-3% of the total system energy, and thus 2.5% was used in the following calculations. The highest values are based on a CW Laser at λ: 532 nm ~ 1064 nm functioning at room temperature (around 20 °C).

Mode Strippers can be utilized for wavelengths beyond this range. However, Fiberguide advises that thorough testing is done to guarantee expected performance. The power handling and temperature limits for such connectors are seen below.  

Connector Type Max. Cladding Energy Dissipated Max. Total Power
MSHP SMA 16.5 W < 125 °C @ Connector 650 W
10 W = 50 °C @ Connector 400 W
MSHP FD-80 18.75 W < 125 °C @ Connector 750 W
12 W = 50 °C @ Connector 480 W

 

Table 1: Background & Assumptions

. .
CW
Air-Silica Fused Interface Damage Threshold
~1.5 MW/cm2 (CW Laser @ λ: 1064 nm)
Damage Threshold is λ dependent, and behaves relatively linearly in the range from 190 nm ~ 2400 nm with the shorter wavelengths being more destructive.
Pulsed
Air-Fused Silica Interface Damage Threshold
~16.0 J/cm2 (Pulsed Laser @ λ: 1064 nm and τ: 1ns)
Damage Threshold is λ dependent, and behaves relatively linearly in the range from 190 nm ~ 2400 nm with the shorter wavelengths being more destructive.
Damage Threshold is τ dependent, and scales with the square root of the pulse duration from 10 ps to 1μs with the shorter pulse durations being more destructive.
NOTE: The CW and Pulsed Air-Fused Silica Interface Damage Thresholds above have been adjusted to compensate for the peak intensity in the Gaussian Beam Profile.
Spot Size Diameter ≤ 85% of the Fiber Core Diameter
Alignment & Beam Waist X & Y Alignment within ± 5% of the core diameter / Z Position beyond source beam waist
Numerical Aperture (NA) Fiber NA ≥ Source NA + 10%
Beam Shape & Quality The spatial profile and quality of the beam will greatly affect high power performance. This analysis assumes a Gaussian Beam where the peak fluence is given by 2 E/p*( Wo)2“, meaning that the peak power is approximately double the 1/e2 specified power.
Connector Polish, End Face, & Flatness The connector end face must be factory laser polished to reduce microscopic inclusions and be cleaned prior to use. The endface must also be flat, <10% of the core diameter peak to valley, so it doesn’t act like a lens and focus the laser energy inside the fiber.
AR Coating None; when AR Coatings are applied to optical fibers, they will always become the limiting factor to power handling capability, so it is important to check the specifics of the selected coating.

 

Please Note: This information provided is designed to help guide product selection, because each optical system is unique, Fiberguide strongly recommends thorough testing before committing to system critical components.

Table 2: CW Power Maximimums for Fiber Core Sizes 100 μm ~ 1500 μm with λ: 193 nm - 2100 nm

Fiber Core Size (μm) Wavelength (nm)
λ: 193 nm λ: 405 nm λ: 532 nm λ: 808 nm λ: 980 nm λ: 1064 nm λ: 1900 nm λ: 2100 nm
100 15 32 43 65 78 85 152 168
200 62 130 170 259 314 340 608 672
300 139 292 383 582 706 766 1368 1512
400 247 518 681 1034 1254 1362 2432 2688
600 556 1166 1532 2327 2822 3064 5472 6048
800 988 2074 2724 4137 5017 5448 9728 10752
1000 1544 3240 4256 6464 7840 8512 15200 16800
1500 3474 7290 9576 14544 17639 19151 34199 37799

 

Table 3: Correction Factors used to Convert Energy Per Pulse to Equivalent Energy Per Pulse at λ: 1064 nm and τ: 1 ns

Pulse Duration(Sec) Wavelength (nm)
λ: 193 nm λ: 405 nm λ: 532 nm λ: 808 nm λ: 980 nm λ: 1064 nm λ: 1900 nm λ: 2100 nm
10 ps 55.13 26.27 20.00 13.17 10.86 10.00 5.60 5.07
50 ps 24.65 11.75 8.94 5.89 4.86 4.47 2.50 2.27
100 ps 17.43 8.31 6.32 4.16 3.43 3.16 1.77 1.60
500 ps 7.80 3.72 2.83 1.86 1.54 1.41 0.79 0.72
1 ns 5.51 2.63 2.00 1.32 1.09 1.00 0.56 0.51
5 ns 2.47 1.17 0.89 0.59 0.49 0.45 0.25 0.23
10 ns 1.74 0.83 0.632 0.42 0.34 0.32 0.18 0.16
50 ns 0.78 0.37 0.28 0.19 0.15 0.14 0.08 0.07
100 ns 0.55 0.26 0.20 0.13 0.11 0.10 0.06 0.05
500 ns 0.25 0.12 0.09 0.06 0.05 0.04 0.03 0.02
1 μs 0.17 0.08 0.06 0.04 0.03 0.03 0.02 0.02

 

Table 4: Maximum Energy Per Pulse (mJ) at λ: 1064 nm and τ: 1 ns for Core Sizes 100 μm ~ 1500 μm

  Fiber Core Size (μm)
100 200 300 400 600 800 1000 1500
Maximum Equivalent Energy Per Pulse (mJ) 0.9 3.6 8.1 14.5 32.6 58.1 90.7 204.2

 

This information has been sourced, reviewed and adapted from materials provided by Fiberguide Industries.

For more information on this source, please visit Fiberguide Industries.

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