Thorlabs Lab Fact: Beamsplitter Package Matters

We present laboratory measurements of the polarization angle, split ratio, and total throughput power of a beam transmitted through Thorlabs plate, cube, and pellicle beamsplitters. While all non-polarizing beamsplitters function similarly, the exact performance is different for different types of beamsplitter. Each type of beamsplitter contains its own advantages and disadvantages compared to other types of beamsplitters. Appropriate choice of beamsplitter is essential to sensitive experimental systems. We present a complete analysis and comparison of optical parameters for three common types of non-polarizing beamsplitters.

For our experiment we used the former generation HRS015 stabilized HeNe laser (replaced by the HRS015B) as the light source for our investigation. A linear polarizer is used to set the laser beam's polarization axis to 45° in order to provide equal S- and P-polarized light incident on the beamsplitter. The beamsplitter under investigation was then placed in the beampath, and its split beams directed to appropriate detectors. The total power though the optic, polarization states, split ratios, and angle of incidence effects were investigated under this configuration.

The plots below summarize the measured results for all three types of beamsplitters. From these graphs the performance of each optic can be easily compared to one another. The bottom left plot summarizes the results for the total power throughput for each optic. The total power throughput is measured as the fraction of input power. While the plate and pellicle beamsplitters perform rather similarly, the cube shows signs of absorption inside the optic. Additionally, this plot shows the relative insensitivity of throughput power to angle of incidence. The bottom middle graph summarizes the results for the output polarization angle for each optic. The cube shows the most similar polarization angles between the reflected and transmitted beams, with the plate producing the largest difference in polarization between beams. The bottom right plot summarizes the results for the split ratio, as a fraction of input power, for the beamsplitters. Here it can be shown that the plate beamsplitter demonstrates the most ideal for 50/50 power splitting. For details on the experimental setup employed and the results summarized here, please click here.

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Beamsplitter Selection Guide

Plate Beamsplitters

Type	Wavelength Range	Splitting Ratio (R:T)	Typical Reflectancea (Click for Plot)	Typical Transmissiona (Click for Plot)	Backside AR Coating	Available Wedged	Available Sizes (Unmounted)
Economy	450 - 650 nm	30:70			No	No	Ø1" and Ø2"
		50:50
UV Fused Silica for UV	250 - 450 nm	50:50			Yes	Yesb	Ø1/2", Ø1", 25 x 36 mm, and Ø2"
UV Fused Silica for UV to NIR	350 - 1100 nm	50:50			Yes	Yesb	Ø1/2", Ø1", 25 x 36 mm, and Ø2"
UV Fused Silica for Visible	400 - 700 nm	10:90			Yes	Yesb	Ø1/2", Ø1", 25 x 36 mm, and Ø2"
		30:70
		50:50
		70:30
		90:10
UV Fused Silica for Visible to IR	600 - 1700 nm	50:50			Yes	Yesb	Ø1/2", Ø1", 25 x 36 mm, and Ø2"
UV Fused Silica for NIR	700 - 1100 nm	10:90			Yes	Yesb	Ø1/2", Ø1", 25 x 36 mm, and Ø2"
		30:70
		50:50
		70:30
		90:10
UV Fused Silica for NIR	1.2 - 1.6 µm	10:90			Yes	Yesb	Ø1/2", Ø1", 25 x 36 mm, and Ø2"
		30:70
		50:50
		70:30
		90:10
IR Fused Silica for IR	0.9 - 2.6 µm	50:50			Yes	Yesb	Ø1/2", Ø1", 25 x 36 mm, and Ø2"
CaF2 for IR	1.0 - 6.0 µm	50:50			No	Yesb	Ø1/2", Ø1", 25 x 36 mm, and Ø2"
	2.0 - 8.0 µm						Ø1/2", Ø1", and Ø2"
ZnSe for IR	1.0 - 12.0 µm	50:50			Yes	Yesb	Ø1"
	7.0 - 14.0 µm						Ø1/2", Ø1", and Ø2"
Laser Line for Nd: YAG	532 nm	50:50			Yes	Yesb	Ø1/2", Ø1", 25 x 36 mm, and Ø2"
	1064 nm
Polka Dot	250 nm - 2.0 µm	50:50	0° and 8° 45°		No	No	Ø1", 1" x 1", and Ø2"
	350 nm - 2.0 µm		0° and 8° 45°		No	No	Ø1", 1" x 1", and Ø2"
	180 nm - 8.0 µm		0° and 12° 45°		No	No	Ø1" and Ø2"
	2.0 - 11.0 µm		0° and 10° 45°		No	No	Ø1"
Ultrafast with Controlled GDDc	600 - 1500 nm	20:80			Yes	No	Ø1"
		50:50
		80:20
		90:10
	1000 - 2000 nm	50:50

Type	Center Wavelength	Extinction Ratio (TP:TS)	Typical Transmissiona (Click for Plot)	Backside AR Coated	Available Wedged	Available Sizes (Unmounted)
Standard	405 nm	>10 000:1		No	Yesb	Ø1" and 25 mm x 36 mm
	532 nm
	633 nm
	780 nm
	808 nm
	1030 nm
	1064 nm
	1310 nm
	1550 nm

• 45° AOI Unless Otherwise Noted
• 30 arcmin Wedge on Round Optics Only
• Designed for use with P-polarized light.

Cube Beamsplitters

Type	Wavelength Range	Splitting Ratio (R:T)	Typical Transmission (Click for Plot)	AR Coated Faces (Click for Plot)	Cemented	Available Cube Side Length
Visible: Unmounted 16 mm Cage Cube 30 mm Cage Cube	400 - 700 nm	10:90			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1"
		30:70			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1"
		50:50			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1", and 2" Mounted: 20 mm in a 16 mm Cage Cube, 1" in a 30 mm Cage Cube
		70:30			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1"
		90:10			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1"
NIR: Unmounted 16 mm Cage Cube 30 mm Cage Cube	700 - 1100 nm	10:90			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm
		30:70			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1"
		50:50			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1", and 2" Mounted: 20 mm in a 16 mm Cage Cube, 1" in a 30 mm Cage Cube
		70:30			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1"
		90:10			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1"
IR: Unmounted 16 mm Cage Cube 30 mm Cage Cube	1100 - 1600 nm	10:90			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm
		30:70			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm
		50:50			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1", and 2" Mounted: 20 mm in a 16 mm Cage Cube, 1" in a 30 mm Cage Cube
		70:30			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1"
		90:10			Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm,1"

Type	Wavelength Range	Extinction Ratio (TP:TS)	Typical Transmission	AR Coated Faces	Cemented	Available Cube Side Length
Standard: Unmounted 16 mm Cage Cube 30 mm Cage Cube	420 - 680 nm	>1 000:1		Yes	Yes	Unmounted: 5 mm, 10 mm, 1/2", 20 mm, 1", and 2" Mounted: 20 mm in a 16 mm Cage Cube, 1" in a 30 mm Cage Cube
	620 - 1000 nm
	900 - 1300 nm
	1200 - 1600 nm
Wire Grid: Unmounted 30 mm Cage Cube	400 - 700 nm	>1 000:1 (AOI: 0° - 5°) >100:1 (AOI: 0° - 25°)	P-Pol. S-Pol.	Yes	Yes	Unmounted: 1" Mounted: 20 mm in a 16 mm Cage Cube, 1" in a 30 mm Cage Cube
High-Power Laser Line: Unmounted 30 mm Cage Cube	355 nm	>2 000:1			No	Unmounted: 1/2" and 1" Mounted: 1" in a 30 mm Cage Cube
	405 nm
	532 nm
	633 nm
	780 - 808 nm
	1064 nm
Laser Line: Unmounted 30 mm Cage Cube	532 nm	>3 000:1		Yes	Yes	Unmounted: 10 mm, 1/2", and 1" Mounted: 1" in a 30 mm Cage Cube
	633 nm
	780 nm
	980 nm
	1064 nm
	1550 nm
Laser-Line Variable	532 nm	Not Specified	No Graph Available	Yes	Yes	Assembly Mounted in a 30 mm Cage Cube
	633 nm
	780 nm
	1064 nm
	1550 nm
Broadband Variable	420 - 680 nm	Not Specified	No Graph Available	Yes	Yes	Assembly Mounted in a 30 mm Cage Cube
	690 - 1000 nm
	900 - 1200 nm
	1200 - 1600 nm
Circular Polarizer/Beamsplitter	532 nm	Not Specified	No Graph Available	Yes	Yes	Assembly Mounted in a 30 mm Cage Cube
	633 nm
	780 nm
	1064 nm
	1550 nm

Pellicle Beamsplitters

Type	Wavelength Range	Splitting Ratio (R:T)	Typical Reflectancea (Click for Plot)	Typical Transmissiona (Click for Plot)	Available Sizes
Pellicle: Unmounted 30 mm Cage Cube	400 - 2400 nm	8:92			Unmounted: Ø1/2", Ø1", and Ø2" Mounted: Ø1" in a 30 mm Cage Cube
	300 - 400 nm	45:55			Unmounted: Ø1" and Ø2" Mounted: Ø1" in a 30 mm Cage Cube
	400 - 700 nm	45:55			Unmounted: Ø1/2", Ø1", and Ø2" Mounted: Ø1" in a 30 mm Cage Cube
	635 nm	33:67			Unmounted: Ø1/2", Ø1", Ø2" Mounted: Ø1" in a 30 mm Cage Cube
	635 nm	50:50			Unmounted: Ø1/2", Ø1", and Ø2" Mounted: Ø1" in a 30 mm Cage Cube
	700 - 900 nm	45:55			Unmounted: Ø1/2", Ø1", and Ø2" Mounted: Ø1" in a 30 mm Cage Cube
	1.0 - 2.0 µm	45:55			Unmounted: Ø1/2", Ø1", and Ø2" Mounted: Ø1" in a 30 mm Cage Cube
	3.0 - 5.0 µm	45:55			Unmounted: Ø1/2" and Ø1" Mounted: Ø1" in a 30 mm Cage Cube

Crystal Beamsplitters

Type	Wavelength Range	Extinction Ratio (TP:TS)	Typical Beam Separation Angle (Click for Representative Diagram)	Typcial Substrate Transmission (Click for Plot)	Typical Coating Reflectance (Click for Plot)	Cemented	Available Sizes
Calcite Beam Displacers	350 nm - 2.3 µm	Not Specified	Parallel with 2.7 mm Spacing at 1500 nm		Uncoated	No	10 mma (Clear Aperture, Square)
			Parallel with 4.0 mm Spacing at 1500 nm				10 mma (Clear Aperture, Square)
Yttrium Orthovanadate Beam Displacer	488 nm - 3.4 µm	Not Specified	Parallel with 1.2 mm Spacing at 2000 nm		Uncoated	No	>3 mm x 5 mmb (Clear Aperture, Ellipse)
	2.0 µm
Wollaston Prisms	400 nm - 2.0 µm	>10 000:1	1° at 633 nm		Uncoated	Yes	Ø10 mma (Clear Aperture)
	200 nm - 6.0 µm	>10 000:1	1° 20' at 633 nm		Uncoated		Ø10 mma (Clear Aperture)
	190 nm - 3.5 µm	>100 000:1	20° at 250 nm		Protective MgF2 (No Data)		Ø10 mma (Clear Aperture)
	350 nm - 2.3 µm	>100 000:1	20° at 633 nm		Uncoated		Ø10 mmb (Clear Aperture)
							Ø10 mma (Clear Aperture)
							Ø10 mma (Clear Aperture)
	900 nm - 3.4 µm	>100 000:1	20° at 633 nm		Uncoated		Ø10 mma (Clear Aperture)
Rochon Prisms	200 nm - 6.0 µm	>10 000:1	1.5° at 4 µm		Uncoated	Yes	10 mm x 10 mma (Min Clear Aperture)
	488 nm - 3.4 µm	>100 000:1	10.6° at 2 µm

• Mounted in a protective box, unthreaded ring, or cylinder.
• Available unmounted or mounted in a protective box or unthreaded cylinder.

Other

Type	Wavelength Range	Splitting Ratio	Substrate Transmission	Typical Reflectance	Available Sizes (Unmounted)
Wedged Plate Beamsplitter	185 nm - 2.1 µm	-		-	2" x 1" (L x H) 8.0 mm (Middle Thickness) Wedge Angle: 5°
Brewster Windows	185 nm - 2.1 µm	-			6.0 mm, 8.0 mm, 13.0 mm, 16.0 mm, 20.0 mm, and 25.0 mm (Minor Diameters)

Laser Induced Damage Threshold Tutorial

The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.

Testing Method

Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.

First, a low-power/energy beam is directed to the optic under test. The optic is exposed in 10 locations to this laser beam for 30 seconds (CW) or for a number of pulses (pulse repetition frequency specified). After exposure, the optic is examined by a microscope (~100X magnification) for any visible damage. The number of locations that are damaged at a particular power/energy level is recorded. Next, the power/energy is either increased or decreased and the optic is exposed at 10 new locations. This process is repeated until damage is observed. The damage threshold is then assigned to be the highest power/energy that the optic can withstand without causing damage. A histogram such as that below represents the testing of one BB1-E02 mirror.

The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm² (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.

According to the test, the damage threshold of the mirror was 2.00 J/cm² (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.

Continuous Wave and Long-Pulse Lasers

When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) [1]. Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.

When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.

Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.

In order to use the specified CW damage threshold of an optic, it is necessary to know the following:

1. Wavelength of your laser
2. Beam diameter of your beam (1/e²)
3. Approximate intensity profile of your beam (e.g., Gaussian)
4. Linear power density of your beam (total power divided by 1/e² beam diameter)

Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below. 

The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).

Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):

While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application. 

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.

Pulsed Lasers

As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.

Pulses shorter than 10^-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism [2]. In contrast, pulses between 10^-7 s and 10^-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.

When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:

1. Wavelength of your laser
2. Energy density of your beam (total energy divided by 1/e² area)
3. Pulse length of your laser
4. Pulse repetition frequency (prf) of your laser
5. Beam diameter of your laser (1/e² )
6. Approximate intensity profile of your beam (e.g., Gaussian)

The energy density of your beam should be calculated in terms of J/cm². The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e² beam.

Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately [3]. A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm² at 1064 nm scales to 0.7 J/cm² at 532 nm):

You now have a wavelength-adjusted energy density, which you will use in the following step.

Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT [4]. For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.

The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:

Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10^-9 s and 10^-7 s. For pulses between 10^-7 s and 10^-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.

[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).