Detectors for Raman spectrometers vary in their sensitivity, spectral range, and cost, and the end user must carefully consider how these parameters will relate to their research needs. Table 1 provides a guide for choosing one of the detectors discussed based on those performance criteria. Selecting the correct detector for a given application will depend on the excitation wavelength needed, the Raman scattering cross-section of the sample, and the desired acquisition time for spectral analysis or imaging.

Raman microscopes most commonly contain charged-coupled device (CCD) detectors and are very sensitive to photon detection.  A CCD is a silicon-based multi-channel one- or two-dimensional array detector that is capable of quickly detecting whole spectra. Each channel within the CCD is a photodiode pixel, of which there are several thousand, and generates electronic charge proportionally to the number of absorbing photons. Therefore, the more photons that reach a CCD pixel in a given time frame, or the longer the photons are allowed to impact the pixel, the more charge is built up and the larger the signal detected. The simplest type of CCD is a full-frame CCD, Figure 1, in which the incoming photons are absorbed on a fully light-sensitive array and the built-up charge is shifted vertically to a readout register before being shifted horizontally to a charge amplifier. Here, the charge is converted to voltage for readout, and at the end of this process, the output can be displayed as a spectral signal on a computer. In a Raman spectrometer, the diffraction grating disperses the scattered light onto the longitudinal axis of the CCD array, meaning that an entire spectrum can be detected in a single acquisition. In two-dimensional CCD arrays, each column corresponds to a wavelength and all of the charges from each pixel in a column are accumulated.

Pulsed Nanosecond Laser Example: Scaling for Different WavelengthsSuppose that a pulsed laser system emits 10 ns pulses at 2.5 Hz, each with 100 mJ of energy at 1064 nm in a 16 mm diameter beam (1/e2) that must be attenuated with a neutral density filter. For a Gaussian output, these specifications result in a maximum energy density of 0.1 J/cm2. The damage threshold of an NDUV10A Ø25 mm, OD 1.0, reflective neutral density filter is 0.05 J/cm2 for 10 ns pulses at 355 nm, while the damage threshold of the similar NE10A absorptive filter is 10 J/cm2 for 10 ns pulses at 532 nm. As described on the previous tab, the LIDT value of an optic scales with the square root of the wavelength in the nanosecond pulse regime:

The mounted aspheric lens that is AR coated for our 1950 nm wavelength and most closely matches the desired focal length of 3.87 mm is our C093TME-D (f = 3.00 mm), shown below. Its clear aperture of 5.00 mm is easily larger than the collimated beam diameter of 1.2 mm. It therefore meets the requirements of the example setup.

IR lensmaterial

BI CCDs, also referred to as back-thinned CCDs, are designed to have improved QE by overcoming the photon flux losses encountered when using FI CCDs. BI CCDs are configured in a process that allows direct exposure of the active photosensitive silicon depletion region to the incoming photons, Figure 7, and therefore they exhibit much higher QEs from the UV to the NIR that reach up to 95%. Although they are not obscured from incoming light by UV-absorbing wires and electrodes, UV photons are absorbed in the surface layers of the semiconductor and are unable to penetrate deeply into the depletion region, which is why the QE of standard BI CCDs decreases at lower wavelengths.

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.

In FI CCDs, incoming light passes through electrode structures and insulating layers located in front of the photosensitive silicon depletion region in which photons are converted into electron-hole pairs to generate charge, Figure 6. The electrodes and insulating layers absorb and reflect a portion of the incoming photons before they reach the silicon, and therefore reduce the QE, which reaches a maximum of between 50% and 60%. This absorbing region is impenetrable to photons with wavelengths of less than 400 nm, which means that standard FI CCDs cannot be used for UV Raman applications.

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.

In an EMCCD, electrons generated from the absorption of photons in the depletion region are accelerated along a multiplication register when voltage is applied. Secondary electrons are generated via impact-ionisation processes in the multiplication register, and the number of electrons produced increases exponentially with voltage. This increase in electrons can be modelled as a function of the number of pixels on the multiplication register and the probability that an electron within each pixel will create a second electron:

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:

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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):

Spherical aberration often prevents a spherical lens from achieving diffraction-limited design. The surfaces of an aspheric lens are designed to minimize spherical aberration, thereby providing a robust single element solution for many applications, such as collimating the output of a fiber or laser diode, coupling light into a fiber, spatial filtering, or imaging light onto a detector. In particular, our IR aspheric lenses are ideal for collimating light from mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) sources, including Quantum Cascade Lasers (QCLs).

The specifications to the right are measured data for Thorlabs' molded IR aspheric lenses made of the Black Diamond-2 (BD-2) material. Damage threshold specifications are constant for all black diamond IR aspheric lenses, regardless of the focal point of the lens. These specifications are limited by the AR coating and are not guaranteed.

Despite the enhancement in QE offered by BI CCDs over FI CCDs, they are prone to significant signal modulation in the NIR caused by constructive and destructive optical interference known as etaloning, Figure 8. This effect was coined because the behaviour described is analogous to a type of interferometer called a Fabry-Pérot étalon.

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The shaded region in each graph indicates the range for which the coating is specified. Please note that these curves are typical; slight variations in performance may occur from lot to lot.

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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.

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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.

Black DiamondBlack Diamond-2 (BD-2), a chalcogenide made of an amorphous mixture of germanium (28%), antimony (12%), and selenium (60%), has several advantages over germanium, which is traditionally used to fabricate IR optics. BD-2's thermally stable refractive index (see the Refractive Index tab) and low coefficient of thermal expansion (13.5 x 10-6 / °C) result in a smaller change in focal length as a function of temperature than for germanium. Additionally, germanium suffers from transmission loss as temperature increases, while BD-2 aspheric lenses can be used in environments up to 130 °C. This material performs particularly well over the 1.7 - 2.2 µm spectral range, providing >99% transmission and a flat dispersion curve. Click here to download a pdf of the SDS for BD-2.

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.

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.

Infrared cameralens

Pulsed Microsecond Laser ExampleConsider a laser system that produces 1 µs pulses, each containing 150 µJ of energy at a repetition rate of 50 kHz, resulting in a relatively high duty cycle of 5%. This system falls somewhere between the regimes of CW and pulsed laser induced damage, and could potentially damage an optic by mechanisms associated with either regime. As a result, both CW and pulsed LIDT values must be compared to the properties of the laser system to ensure safe operation.

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However, the maximum power density of a Gaussian beam is about twice the maximum power density of a uniform beam, as shown in the graph to the right. Therefore, a more accurate determination of the maximum linear power density of the system is 1 W/cm.

The aspheric surfaces of these lenses may be described using a polynomial expansion in Y, the radial distance from the optical axis. The surface profile or sagitta (often abbreviated as sag) is denoted by z, and is given by the following expression:

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.

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.

CW Laser ExampleSuppose that a CW laser system at 1319 nm produces a 0.5 W Gaussian beam that has a 1/e2 diameter of 10 mm. A naive calculation of the average linear power density of this beam would yield a value of 0.5 W/cm, given by the total power divided by the beam diameter:

Unsure whether a CCD, EMCCD, or InGaAs detector is best for your Raman application? In this Spectral School tutorial, we discuss the differences between the detectors available for Raman microscopes, and how to select one based on important performance parameters such as quantum efficiency and spectral range.

The ability of a CCD to detect a photon is characterised by a parameter called quantum efficiency (QE), which is a ratio between the number of electrons generated and the number of photons absorbed and is a key indicator of detector sensitivity. The QE of a detector varies with wavelength, and the result of a plot of the two variables is a QE curve that characterises the performance of the detector across its operational wavelength range, Figure 2.  Each detector has a unique QE curve, which means that the sensitivity and spectral range of different detectors can easily be compared and evaluated with respect to the user’s research requirements.

Due to the semiconductor materials and the manufacturing processes used to make standard FI and BI CCDs, the QE of both detector types are compromised in the UV region of the electromagnetic spectrum. The insulating region and electrodes that cover the depletion region in FI CCDs are opaque to light with wavelengths less than 400 nm, and the first few surface layers of the depletion region absorb UV light strongly in BI CCDs. Physical and chemical changes are therefore needed to improve the QE of both CCD types at lower wavelengths, such as passivating the semiconductor material to reduce charge-carrier recombination and applying down-conversion coatings on the surface to increase depth penetration. Such enhancements are available for FI and BI CCDs, Figure 10.

Infraredlensphotography

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.

The QE of CCDs drops off sharply in the IR and as a result, they cannot be used to detect light with wavelengths greater than 1100 nm. This is because the silicon in the depletion region cannot absorb photons that are lower than its bandgap energy. Spectral detection capabilities can be extended beyond this limit in a Raman spectrometer by using an alternative type of detector based on a material called indium gallium arsenide (InGaAs), a semiconductor alloy of gallium arsenide and indium arsenide that has a lower bandgap energy than silicon and exhibits excellent photosensitivity in the NIR and shortwave IR range. Typically, InGaAs detectors exhibit QEs of above 80% between 1000 nm and 1600 nm and are sensitive up to 1700 nm, Figure 11, greatly outperforming silicon CCDs and making them essential for Raman applications that utilise a 1064 nm laser.

Despite being very sensitive to photon detection, CCDs are limited in applications involving small numbers of photons because they cannot deliver both high sensitivity and rapid acquisition speeds simultaneously. This is because, in a CCD, readout noise scales with the speed at which the charge is amplified, which reduces the sensitivity of detection. This is especially noticeable when using samples that have small Raman scattering cross sections, are low in concentration, or require very fast spectral acquisition times. Electron-multiplied CCDs (EMCCDs) are an alternative type of detector without this limitation and hence they offer ultrahigh sensitivity and speeds. EMCCDs utilise a frame-transfer CCD structure in which the photons are captured in an image section and the resulting charge is temporarily stored in a parallel storage section. Crucially, they are also equipped with multiplication gain technology to increase the number of electrons generated from a single photon, Figure 3. This occurs before the charge reaches the charge amplifier, so the signal is boosted significantly in comparison to noise. They are the best option for fast mapping or for imaging samples that have low levels of Raman scattering.

The two types of CCDs and EMCCDs available are front-illuminated (FI) and back-illuminated (BI), named in terms of the direction incident photons interact with the detector. These detectors differ significantly in their structures, and therefore they have very different QE curves. As can be observed in the example QE plots in Figure 5, a BI CCD offers a higher QE and hence sensitivity than a FI CCD across the entire range that the silicon is photoactive.

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):

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The adjusted LIDT value of 350 W/cm x (1319 nm / 1550 nm) = 298 W/cm is significantly higher than the calculated maximum linear power density of the laser system, so it would be safe to use this doublet lens for this application.

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.

For optimal coupling, the spot size of the focused beam should be smaller than the MFD of the single mode fiber. Therefore, if an aspheric lens is not available that provides an exact match, choose an aspheric lens with a focal length that is shorter than that yielded by the calculation above. Alternatively, assuming the clear aperture of the aspheric lens is sufficiently large, the beam can be expanded before the aspheric lens to allow the focused beam to have a tighter spot.

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All of the molded glass lenses featured on this page are available with an antireflection coating for either the 1.8 - 3 µm, 3 - 5 µm, or 8 - 12 µm range deposited on both sides. Other AR coating options are listed in the Aspheric Lens Selection Guide table at right.

VIG06VIG06 is a chalcogenide made of an amorphous mixture of arsenic (40%) and selenium (60%). VIG06 has similar optical properties to BD-2, but with a slightly higher refractive index and coefficient of thermal expansion (20.8 x 10-6 / °C) and a lower thermooptic coefficient (32.1 x 10-6 / °C). VIG06 aspheric lenses can be used in environments with temperatures up to 110 °C. Click here to download a pdf of the SDS for VIG06.

Ophirlens

This scaling gives adjusted LIDT values of 0.08 J/cm2 for the reflective filter and 14 J/cm2 for the absorptive filter. In this case, the absorptive filter is the best choice in order to avoid optical damage.

As described above, the maximum energy density of a Gaussian beam is about twice the average energy density. So, the maximum energy density of this beam is ~0.7 J/cm2.

In order to illustrate the process of determining whether a given laser system will damage an optic, a number of example calculations of laser induced damage threshold are given below. For assistance with performing similar calculations, we provide a spreadsheet calculator that can be downloaded by clicking the button to the right. To use the calculator, enter the specified LIDT value of the optic under consideration and the relevant parameters of your laser system in the green boxes. The spreadsheet will then calculate a linear power density for CW and pulsed systems, as well as an energy density value for pulsed systems. These values are used to calculate adjusted, scaled LIDT values for the optics based on accepted scaling laws. This calculator assumes a Gaussian beam profile, so a correction factor must be introduced for other beam shapes (uniform, etc.). The LIDT scaling laws are determined from empirical relationships; their accuracy is not guaranteed. Remember that absorption by optics or coatings can significantly reduce LIDT in some spectral regions. These LIDT values are not valid for ultrashort pulses less than one nanosecond in duration.

Aspheric lenses are commonly used to couple incident light with a spot size of 1 - 5 mm into a single mode fiber. The following simple example illustrates the key specifications to consider when trying to choose the correct lens.

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.

There are several approaches for reducing etaloning in BI CCDs. The first is the application of an anti-reflective (AR) coating to increase the transmissivity of the silicon-air interface, Figure 9B. When NIR photons enter the silicon region in a BI CCD and propagate towards the insulating layer without being absorbed, they will be reflected towards the front surface with the silicon-air interface. If this front surface is AR coated, the number of photons that undergo further internal reflection and contribute to signal modulation in the silicon is reduced. Another approach is to roughen the back surface of the silicon region during the manufacturing process, Figure 9C. This is referred to as fringe suppression and it helps to reduce etaloning because it breaks the parallelism of the two reflective interfaces and thus reduces constructive interference. Finally, the depth of the silicon depletion region can be increased, Figure 9D. The benefit of a deep depletion region is that NIR photons have a higher probability of being absorbed before multiple internal reflections occur. BI CCDs can be manufactured with all three features to reduce interference patterns in spectra and increase QE in the NIR, but it should be noted that etaloning can never be fully removed.

Due to the rotational symmetry of the lens surface, only even powers of Y are contained in the polynomial expansion above. The target values of the aspheric coefficients for each product can be found by clicking either on the blue Info Icons in the tables below () or on the red documents icon () next to each lens sold below.

In a Raman microscope, the role of the detector is to convert photons into a meaningful signal that provides qualitative and quantitative information about the molecular structure of the sample under investigation. Several types of detectors are available, and they can all be evaluated in terms of their sensitivity to incident photons and the spectral range over which they are operational. An ideal detector would convert every photon to an output signal, but in practice, the functionality of a detector is typically limited to a specific spectral region that is dictated by its constituent materials and the manufacturing process. Since Raman spectroscopy can be applied to an extensive range of samples with widely varying optical properties, the consideration and careful selection of a detector before commencing a research project are vital to the optimisation of later results. In this Spectral School tutorial, we will discuss the different types of detectors available for Raman microscopes and how the sensitivity and spectral range required by the user will dictate their choice of detector.

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.

The energy density of the beam can be compared to the LIDT values of 1 J/cm2 and 3.5 J/cm2 for a BB1-E01 broadband dielectric mirror and an NB1-K08 Nd:YAG laser line mirror, respectively. Both of these LIDT values, while measured at 355 nm, were determined with a 10 ns pulsed laser at 10 Hz. Therefore, an adjustment must be applied for the shorter pulse duration of the system under consideration. As described on the previous tab, LIDT values in the nanosecond pulse regime scale with the square root of the laser pulse duration:

LIDT in linear power density vs. pulse length and spot size. For long pulses to CW, linear power density becomes a constant with spot size. This graph was obtained from [1].

At 1950 nm, Thorlabs' P1-1950-FC-1 single mode patch cable is specified with a mode field diameter (MFD) of 8.0 μm. This specification should be matched to the diffraction-limited spot size given by the following equation:

The penetration depth of light in the photosensitive silicon increases with wavelength, to the point where the material is semi-transparent in the NIR, and the consequence of this is that NIR light can propagate through large thicknesses before being absorbed. This penetration depth is several times the thickness of the ultrathin photosensitive silicon of the BI CCD, and because there are large refractive index differences and hence reflectivity at both interfaces (insulating layer-silicon and silicon-air) of this region, NIR photons can undergo back-and-forth internal reflection which generates interference, Figure 9A. It is important to note that etaloning will not occur when visible excitation wavelengths are used, but it can become more prominent when 785 nm or longer wavelength excitation sources are used, especially when the sample under investigation is reflective or requires high acquisition times because it is a weak Raman scatterer. FI CCDs do not suffer from this effect, because incident NIR photons are not reflected into the depletion region by a reflective interface. Instead, the photons that are not absorbed in the silicon are lost in an optically dead bulk substrate.

If this relatively long-pulse laser emits a Gaussian 12.7 mm diameter beam (1/e2) at 980 nm, then the resulting output has a linear power density of 5.9 W/cm and an energy density of 1.2 x 10-4 J/cm2 per pulse. This can be compared to the LIDT values for a WPQ10E-980 polymer zero-order quarter-wave plate, which are 5 W/cm for CW radiation at 810 nm and 5 J/cm2 for a 10 ns pulse at 810 nm. As before, the CW LIDT of the optic scales linearly with the laser wavelength, resulting in an adjusted CW value of 6 W/cm at 980 nm. On the other hand, the pulsed LIDT scales with the square root of the laser wavelength and the square root of the pulse duration, resulting in an adjusted value of 55 J/cm2 for a 1 µs pulse at 980 nm. The pulsed LIDT of the optic is significantly greater than the energy density of the laser pulse, so individual pulses will not damage the wave plate. However, the large average linear power density of the laser system may cause thermal damage to the optic, much like a high-power CW beam.

Here, G is the gain, P is the probability of gain occurring and it ranges between 0.01 and 0.016 depending on the voltage applied and the temperature of the detector, and finally, N is the number of pixels. It should also be noted an EMCCD can be operated with adjusted gain levels and even without gain, as is shown in the example spectra in Figure 4, in which case it would compare in performance to a standard CCD.

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where R is the radius of curvature, k is the conic constant, and the An are the nth order aspheric coefficients. The sign of R is determined by whether the center of curvature for the lens surface is located to the right or left of the lens' vertex; a positive R indicates that the center of curvature is located to the right of the vertex, while a negative R indicates that the center of curvature is located to the left of the vertex. For example, the radius of curvature for the left surface of a biconvex lens would be specified as positive, while the radius of curvature for its right surface would be specified as negative.

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IR lensglasses

Pulsed Nanosecond Laser Example: Scaling for Different Pulse DurationsSuppose that a pulsed Nd:YAG laser system is frequency tripled to produce a 10 Hz output, consisting of 2 ns output pulses at 355 nm, each with 1 J of energy, in a Gaussian beam with a 1.9 cm beam diameter (1/e2). The average energy density of each pulse is found by dividing the pulse energy by the beam area:

[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).

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).

IR lensfilter

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.

If an unmounted aspheric lens is being used to collimate the light from a point source or laser diode, the side with the greater radius of curvature should face the point source or laser diode. To collimate light using one of our mounted aspheric lenses, orient the housing so that the externally threaded end of the mount faces the source.

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.

An AC127-030-C achromatic doublet lens has a specified CW LIDT of 350 W/cm, as tested at 1550 nm. CW damage threshold values typically scale directly with the wavelength of the laser source, so this yields an adjusted LIDT value:

LIDT in energy density vs. pulse length and spot size. For short pulses, energy density becomes a constant with spot size. This graph was obtained from [1].

These molded glass lenses are available unmounted or premounted in stainless steel lens housings that are engraved with the part number for easy identification. These housings have a metric external threading that makes them easy to integrate into an optical setup or OEM application. For example, they are readily adapted to our SM1 (1.035"-40) Lens Tubes by using our Aspheric Lens Adapters. Mounted aspheres can also be used as a drop-in replacement for multi-element microscope objectives in conjunction with our RMS-threaded Objective Replacement Adapters.

This adjustment factor results in LIDT values of 0.45 J/cm2 for the BB1-E01 broadband mirror and 1.6 J/cm2 for the Nd:YAG laser line mirror, which are to be compared with the 0.7 J/cm2 maximum energy density of the beam. While the broadband mirror would likely be damaged by the laser, the more specialized laser line mirror is appropriate for use with this system.

The refractive indices of Black Diamond-2 (BD-2) and VIG06 as a function of wavelength, shown above, was calculated using the Herzberger Equation, an infrared-specific analog of the Sellmeier Equation. The Herzberger coefficients for BD-2 and VIG06 are given to the table to the right.

Here, f is the focal length of the lens, λ is the wavelength of the input light, and D is the diameter of the collimated beam incident on the lens. Solving for the desired focal length of the collimating lens yields:

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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.