2.1.

Instrumental Setup

The schematic of the experiment is shown in Fig. 2. The sensors under test were irradiated by two different types of laser systems to perform the laser-induced damage tests:

Fig. 2

Experimental setup: (a) configuration for CCD/CMOS damaging testing and (b) configuration for the DMD testing.

The incident laser energy of the pulsed laser was adjusted by varying the time delay between the beginning of the flash lamp pulse and the trigger of the Pockels cell. The power level of the CW-laser source was controlled by changing the current of the diode driver from 49% up to 100%.

Furthermore, we used a set of neutral density filters with different optical densities (ODs) ranging from OD 0.5 to OD 3.0. To control different exposure times of the CW-laser source, we used a laser shutter (Uniblitz Shutter Systems VS25). Finally, the laser beam was focused by a lens (Apo-Rodagon N 4.0/80, Qioptiq) with a focal length of $f=80\mathrm{mm}$ and an aperture of $f/5.6$, thus the beam diameter in the focal plane was measured by the beam profiler BP209 from Thorlabs using a scanning slit method. The measurement with the beam profiler took place before the actual experiment. Therefore, we were able to determine the diameter ($1/e^2$) of the laser spot in the focal plane and got a value of $2{\omega }_1=25.7\mu \mathrm{m}$ for the CW-laser source and $2{\omega }_2=28.2\mu \mathrm{m}$ for the pulsed laser source. The effective diameter $d_{\mathrm{eff}}$ of a uniform cylindrical beam with the same peak intensity and total power as a cylindrical Gaussian beam is¹⁵

The device under test was installed on a 3-D translation stage, so that we could shift the sensor into the focal plane of the lens and each pulse could be exposed to an unused test site (Fig. 2). To detect any changes in the image of the camera, we observed the output signal of the camera on the computer. The experimental setup for the investigation concerning the DMD differed from those concerning the CMOS/CCD sensors. The DMD was also put in the focal plane, but in contrast to the experiments concerning the CMOS/CCD cameras, we used an observer camera [camera 2 in Fig. 2(b), VRmagic VRmFC-22/BW] outside the optical axis to record the position of the laser spot on the DMD and a second observing camera [camera 1 in Fig. 2(b), The Imaging Source DFK22AUC03], which could be flipped into the optical axis, so we could take a picture of the DMD before and after each laser shot to detect whether damage occurred or not.

To compare the output images of the cameras before and after each shot, we illuminated the cameras as uniformly as possible. For that purpose, we used an integrating sphere (connected to a stabilized fiber-coupled light source SLS201/M from Thorlabs), which could be flipped into the beam path in front of the lens. In the case of the DMD sensor, we illuminated the sensor from outside the optical axis with a halogen cold light source (Schott KL 255 LCD) so that the center of the DMD is illuminated as uniformly as possible. Note that the micromirrors’ normal and the optical axes form an angle of $+12\mathrm{deg}$ to the beam axis in the “on” position and an angle of $-12\mathrm{deg}$ in the “off” position with the result that in one position the light was reflected to observing camera 1 and in one it was not.

The experiment described was performed using monochromatic and color CMOS cameras including an imaging sensor Aptina MT9V024 with a resolution ($\text{horizontal}\times \text{vertical}$) of $744\times 488\text{pixels}$ and a pixel size of $6\mu \mathrm{m}\times 6\mu \mathrm{m}$. We also examined the damage formation of monochromatic and color CCD cameras operating with an imaging sensor (monochromatic: Sony ICX098BQ, color: Sony ICX098BL) at a resolution of $640\times 480\text{pixels}$ and a pixel size of $5.8\mu \mathrm{m}\times 5.8\mu \mathrm{m}$. The third device under test was a DMD with $1024\times 768$ micromirrors (Texas Instruments DLP7000). All the investigated samples were encapsulated with a protecting glass plate, so we did not expect any contamination (e.g., dust particles) directly on the bare imaging sensor. It can, therefore, be considered that there is no influence on the measured damage thresholds by contamination. Of course, impurities in the starting materials used in sensor production cannot be excluded. The distance from the surface of the cover glass to the surface of the imaging sensor was about $1.94\pm 0.15\mathrm{mm}$ in case of the CMOS and CCD cameras and the distance from the surface of the cover glass to the surface of the mirror array was about $2.9\pm 0.1\mathrm{mm}$ in case of the DMD. The distance corresponded approximately to the double of the Rayleigh range. This means that the laser power density or the laser pulse energy density on the glass surface was significantly lower than the one on the surface of the imaging sensor. All samples are listed in Table 1.

Table 1

Specifications of the samples under test.

2.2.

Implementation of the Experiment

The experiments did not take place in a classified clean room. Therefore, we had to ensure that the protecting glass plates of the imaging sensors were clean from harmful dust and dirt before each test procedure. The test site of the investigated object was positioned in the focal plane. To measure the damage threshold, we used the 1-on-1 test mode, so each test site was irradiated by one pulse or in the case of CW-laser radiation for a certain exposure time (0.25, 1, 5, and 10 s). Before each laser shot, we took two reference images: an illuminated image (flat-field) and a dark-frame with the same exposure time as we used taking the raw image. In the following, we exposed the sensor with a highly attenuated laser beam, so that no damage could occur to the sensor. We captured a picture during this laser irradiation with the effect that we could determine the positon of the impact of the laser beam on the sensor. After the irradiation with high-power/pulse energy, we took an unexposed image (dark image) and an illuminated image (bright image). Finally, we changed the test site equally whether the sample was damaged or not and repeated the whole procedure after increasing the laser power/pulse energy. Figures 3 and 4 show a selection of raw images of the inflicted damage with and without illumination.

Fig. 3

Raw images of various monochrome and color cameras showing pulsed laser-induced damages in the (a) dark images and (b) in the bright images. The laser fluence used to cause damage increases from top to bottom.

Fig. 4

Raw images of various monochrome and color cameras and the DMD showing pulsed laser-induced damages in the bright images. The laser power density used to cause damage increases from left to right.

Fig. 5

Steps of image processing in case of an illuminated camera image (bright image) and an unexposed camera image (dark image).

3.1.

Image Processing

In the case of pulsed laser irradiation, the damaged area in dark images and bright images appeared in the same shape. For the sake of simplicity, we used the dark images to evaluate the damage threshold. We cut the corresponding area first and then subtracted each image with a reference dark-frame (see Fig. 5). In the case of CW-laser irradiation, we used the bright images for evaluation, because there was no damage visible in the dark images. We corrected each raw image by flat-field correction. This was done by removing the DC offset signal by subtracting the dark-frame from the raw image and multiplying it with the normalized flat-field correction image.¹⁶ Color images were converted into grayscale. Finally, we defined damage as a 10% deviation from intensity of the normalized and scaled reference image. Based on this threshold, we converted the postcorrection image into a binary image, so that all pixel values below the threshold value are set to 0 and those above this value to 1. Afterward, we drew a contour line around all pixels with value 1 and, finally, we calculated the area inside this contour.

As long as the laser damage resembles a circular disc, this method is quite easy to apply, see Fig. 5(c). However, for larger laser powers/pulse energies, some deviations can occur:

In the case of single line damage, the contour line is a convex hull around all pixels with the value 1 [see yellow line in Fig. 5(b)]. In the case of star-shaped damage, including line damage, the damaged area was also marked by contour lines. Additionally, the mean distance from the position of the laser spot center on the surface to each outer edge right beside the line damage was determined. After that, a mean circle was computed into the image [see thin red line in Fig. 5(a)]. We defined the damaged area as all pixels that were contained either within the contour or within this mean circle.

3.2.

Estimation of the Damage Threshold Based on Thermal-Induced Damages

A logarithmic relationship between the damaged area and laser pulse energy is used to determine the damage threshold and is described in various other experiments.^17–20 In their work, the authors observed by microscope images the evolution of a series of concentric rings, generated through the exposure of ultrashort laser pulses, which they called a specific amorphous ring pattern. Apparently, the ring formation is associated with photo-thermal damage of the subject due to laser radiation. The spatial intensity distribution of a Gaussian laser beam is described as

In the case of pulsed lasers, the laser-induced damage threshold (LIDT) of absorbing materials is a constant value if measured in terms of $\mathrm{J}/{\mathrm{cm}}^{-2}$ so $\mathrm{\varphi }$ corresponds to the applied laser fluence $F$, and in the case of CW-laser radiation, the LIDT is a constant value if measured in $\mathrm{W}/{\mathrm{cm}}^{-2}$ so $\mathrm{\varphi }$ corresponds to the irradiance. The distance $r_i$ is the outer radius of the damaged area.²¹ The relationship of Eq. (4) also applies in the case of the ablation caused by nanosecond pulses.²² Following this idea, we plotted the area of the damaged surface of the detector material in pixels versus the fluence or the power density in a semilog plot, respectively, and fitted straight lines to the data according to Eq. (4). The only difference from other experiments is that we did not look at the physically damaged area of the detector material. Moreover, we were interested in the laser-induced damage, which became visible in the output camera image. We defined the “reconstructed beam diameter” (RBD) as the beam diameter we got from the slope of Eq. (4). It represents the required beam diameter of the laser source on the surface of the test object, if the expansion of disturbance in the camera image resembled the physical damage in the sensor.

For pulsed laser radiation, we identified four different types of laser-induced impact on the cameras, which we classified in four groups (see Table 2). Each group was marked by vertical blue dashed lines in the graphs of Fig. 6Fig. 7–8. In the case of CW-radiation, only spot damage and in a few occasions line damage occurred.

Table 2

Groups of laser-induced impact on the devices

Fig. 6

Damaged area size of a monochrome CMOS camera as a function of pulsed laser fluence in semilog plot: (a) full data and (b) section of the full data consisting damage types I and II. The vertical blue dashed lines demarcate different stages of damage phenomenon.

Fig. 7

Size of the damaged area of a color CMOS camera as a function of pulsed laser fluence in semilog plot: (a) full data and (b) section of the full data consisting damage types I and II. The vertical blue dashed lines demarcate different stages of damage phenomenon.

Fig. 8

Size of the damaged area of a color CCD camera as a function of pulsed laser fluence in a semilog plot. The blue dashed lines demarcate different stages of damage phenomenon.

3.3.

Observation, Morphology, and Threshold of Pulsed Laser-Induced Damage on CMOS Cameras

In the case of the monochrome CMOS cameras exposed to pulsed laser light, first sensor damages were observed at a fluence of $0.1\mathrm{J}/{\mathrm{cm}}^2$. No damage was observed below a level of $0.043\mathrm{J}/{\mathrm{cm}}^2$. The damages appear as white “hot pixels” in the dark and in the bright images [see Fig. 3(1–3)]. This may indicate that pulsed laser radiation causes a change in the bandgap of the semiconductor or a change in the insulation area resulting in an increasing leakage current. Whether there is incident light or not, white pixels are quite apparent. The shape of the damage was mainly circular and slightly elongated in the horizontal direction. At further increased fluence, the spot damage started to develop star-shaped edges around a circular center, starting from a value of $14.9\mathrm{J}/{\mathrm{cm}}^2$. In Fig. 3([3b]), such peaks can also be recognized at fluence levels at which line damage occurs, but not to a great extent. Line damage, an emergence indicating that a whole line in the horizontal and/or in vertical direction became inoperative, started at a value of about $1.42\mathrm{J}/{\mathrm{cm}}^2$. In the vertical direction, the line damage extended steadily to the borders of the detector. At high fluence values, the line damage on the right-hand side of the damaged area was stronger than the line damage on the left-hand side. Line damage may be caused by signal interruption because of the device circuit fuses being cut. There was no visible difference in the shape of the damaged area for the bright images and the dark-frame. The graph in Fig. 6(a) shows the damaged area as a function of pulsed laser fluence for the monochrome CMOS camera. We performed two linear fits [cf. Eq. (4)] to the data with different slopes. For the fit concerning the data of groups I and II [see the red line in Fig. 6(b)], we estimated a damage threshold of $F_{\mathrm{th}}=(0.076\pm 0.019)\mathrm{J}/{\mathrm{cm}}^2$ and a corresponding RBD of $2{\omega }_0=(18.9\pm 3.8)\mu \mathrm{m}$. This estimated value corresponds to the previously measured beam diameter, so we can assume that the physical damage of the sensor and the disturbance in the corresponding output image are comparable. As soon as the spot damage starts to develop as star shaped, the linear fit to the data becomes a different slope [see green line in Fig. 4(a)], which would result in another RBD of $2{\omega }_0=(1285\pm 5)\mu \mathrm{m}$. Consequently, we can conclude that the disturbance in the output image of the camera grows faster than the physical damage on the sensor chip. We interpreted the intersection of the green and the red line as the beginning of the formation of the star-shaped character at a fluence value of $F_s=47.2\mathrm{J}/{\mathrm{cm}}^2$.

The formation of damage by irradiating color CMOS cameras started to grow at a level of $F=0.053\mathrm{J}/{\mathrm{cm}}^2$. No damage was observed below $0.043\mathrm{J}/{\mathrm{cm}}^2$. Line damage was observed at a level of $F=3.7\mathrm{J}/{\mathrm{cm}}^2$. Star-shaping started at a value of $38.6\mathrm{J}/{\mathrm{cm}}^2$. The shape of the spot damage was also circular and slightly elongated in the horizontal direction. No difference in the composition of the damage in dark and the bright images was observed. It is striking that the damaged pixels appeared predominantly green, both in the dark image and in the bright image [see Fig. 3(4–5)]. Only in the case of higher pulse energies, did the damaged pixels appear mainly white in the center of the damaged area and green at the edges of the area in the dark image. It is remarkable that in the flat-field image, the edges of the damaged area appeared in red [see Fig. 3(6)]. The line damage represented itself in red, yellow, and black lines or in blue and black lines for lower energies. Only in case of higher energy, was there a bunch of black lines with blue lines at one outer border and red as well as yellow lines at the other border. The camera sensor was completely destroyed in the sense that the output image no longer reacted to incident light, at a level of $2.9\mathrm{kJ}/{\mathrm{cm}}^2$. The red curve in Fig. 7 represents the dependence of the damaged area versus input fluence for low laser pulse energy. From the linear fit, we estimate a damage threshold of $F_{\mathrm{th}}=(0.035\pm 0.009)\mathrm{J}/{\mathrm{cm}}^2$ and an RBD of $2{\omega }_0=(14.9\pm 4.5)\mu \mathrm{m}$. The intersection of the green and the red curve lies at a fluence value of $F_s=102\mathrm{J}/{\mathrm{cm}}^2$. The fact that the damage threshold of the color CMOS camera was lower than the damage threshold of the monochromatic device is an indication that the first damage in color cameras emerges in the Bayer filter.

3.4.

Observation, Morphology, and Threshold of Pulsed Laser-Induced Damage on CCD Cameras

For monochrome CCD cameras, the formation of damage started at a fluence of $F=0.032\mathrm{J}/{\mathrm{cm}}^2$. No damage was observed below a level of $0.004\mathrm{J}/{\mathrm{cm}}^2$. Line damage was also observed and started at a fluence value of $F=0.35\mathrm{J}/{\mathrm{cm}}^2$. At a fluence value of $F=147\mathrm{J}/{\mathrm{cm}}^2$, the whole sensor was broken. The damage appeared as a white “hot pixel” in the dark image as well as in the bright image [Fig. 3(7–9)]. Line damage evolved only in the vertical direction. The damage on the camera started as point damage and, in contrast to CMOS cameras, the damage elongated in the vertical direction as the energy increased, starting at a fluence of $F=0.14\mathrm{J}/{\mathrm{cm}}^2$. This behavior was also observed in other works.²³ For most CCD cameras, the electrodes in each pixel are arranged in such a way that the charge is transferred in the vertical direction along the column to the final row (readout register). To avoid the charges escaping laterally, there are some “channel-stops” implanted near to the surface to isolate the charge packets from adjacent columns. In the case of strong irradiation, the created charge carriers prefer the vertical direction. The shape of the damage was equal in both the dark-frame and flat-field images. Due to the destruction of the sensor, we did not receive enough data to perform linear curve-fitting.

In the case of the color CCD camera, at first glance the bright image exhibited a different red level baseline after each shot. The formation of spot damage started at a level of $F=0.034\mathrm{J}/{\mathrm{cm}}^2$ and no damage was observed beneath a fluence value of $0.014\mathrm{J}/{\mathrm{cm}}^2$. Line damage occurred at a level of $F=0.49\mathrm{J}/{\mathrm{cm}}^2$ and appeared red, blue, or yellow. At a level of $3.16\mathrm{J}/{\mathrm{cm}}^2$, the camera was destroyed. The damage occurred almost circularly above a fluence of $0.064\mathrm{J}/{\mathrm{cm}}^2$ and appeared green in the dark image and yellow in the bright image. The damage exhibited the same shape in the bright and the dark images. Due to the red background in the image, the damaged pixels, which are related to the green channel of the sensor, appeared in yellow. The damage shape elongated in the vertical direction with increasing energy. At higher laser energy, the color of the damaged area turned mostly white, but at the outer border of the damage, all colors are represented. From the fit in Fig. 8, we got a damage threshold $F_{\mathrm{th}}=(0.041\pm 0.003)\mathrm{J}/{\mathrm{cm}}^2$ and an RBD of $2{\omega }_0=(37\pm 5)\mu \mathrm{m}$.

3.5.

Observation, Morphology, and Threshold of CW-Laser-Induced Damage on CMOS Cameras

In contrast to pulsed laser-induced damage, no visible damage occurs in the dark image. Therefore, only the bright images were used to analyze the visible damage. First damage occurred in case of the monochrome CMOS cameras for exposure times of 0.25, 1, 5, and 10 s at a power density of 85, 85, 57, and $49\mathrm{kW}/{\mathrm{cm}}^2$, respectively. The shape of the damage was mostly circular and slightly blurred, because the damaged pixel became less sensitive but did not fail completely. The damage appeared dark in the flat-field image in opposition to the pulsed-laser damage, where the damage appeared white in the flat-field image. We also observed line damage starting from a power density of $196\mathrm{kW}/{\mathrm{cm}}^2$. From the fit in Fig. 9(a), we got a damage threshold depending on the exposure time of $F_{\mathrm{th}}=[75\pm 7,73\pm 13,56\pm 4,48\pm 3]\mathrm{kW}/{\mathrm{cm}}^2$. Obviously, the slope of the lines in red and black are different from those in green and blue. According to Eq. (4), different slopes are associated with different RBD $2{\omega }_0=[14\pm 5,16\pm 4,20\pm 3,21\pm 3]\mu \mathrm{m}$.

Fig. 9

Size of the damaged area of a CMOS camera versus power density of a CW-laser in a semilog plot: (a) monochrome camera and (b) color camera.

In the case of the color CMOS cameras, damage started at a power density of $46\mathrm{kW}/{\mathrm{cm}}^2$ for an exposure time of 0.25 s. No line damage was observed. Damaged pixels seemed almost purple, in other words, a combination of blue and red pixel values. Just as it was in the case of damage to the monochromatic device, the shape appeared almost circular and blurred. From the fit in Fig. 9(b), we got a damage threshold of $F_{\mathrm{th}}=(56.7\pm 1.8)\mathrm{kW}/{\mathrm{cm}}^2$ and an RBD $2{\omega }_0=(12.6\pm 0.5)\mu \mathrm{m}$.

3.6.

Observation, Morphology, and Threshold of CW-Laser-Induced Damage on CCD Cameras

For the monochrome CCD camera, it was quite challenging to cause damage to the sensor. Damage started to occur for exposure times of 0.25, 1, 5, and 10 s at power densities of 163, 139, 139, and $139\mathrm{kW}/{\mathrm{cm}}^2$, respectively. No damage occurred below a value of $135\mathrm{kW}/{\mathrm{cm}}^2$ for exposure times of 1, 5, and 10 s and below a value of $159\mathrm{kW}/{\mathrm{cm}}^2$ for an exposure time of 0.25 s. The shape of damage is almost circular and the damaged pixels are dark in the output image. From the fit in Fig. 10(a), we got a damage threshold of $F_{\mathrm{th}}=[146\pm 9,118\pm 9,93\pm 19,95\pm 23]\mathrm{kW}/{\mathrm{cm}}^2$ and an RBD $2{\omega }_0=[22\pm 3,27\pm 3,25\pm 3,25\pm 3]\mu \mathrm{m}$, respectively.

Fig. 10

Size of the damaged area of a CCD camera versus power density of a CW-laser in a semilog plot: (a) monochrome camera and (b) color camera.

Fig. 11

Size of the damaged area of the DMD versus power density of a CW-laser source.

In case of the color CCD camera, we observed the same behavior as in case of the pulsed-laser-induced damage to the camera of the same type. The bright image exhibited a different red baseline after each shot. First damage for exposure times of 0.25, 1, 5, and 10 s started at a power density of 16, 16, 9, and $9\mathrm{kW}/{\mathrm{cm}}^2$ and no damage occurred below a level of 9.2, 9.2, 5.5, and $5.5\mathrm{kW}/{\mathrm{cm}}^2$. No line damage was observed. The damage shape was circular. Damaged pixels were almost purple or deep red because of the high red levels. The information of the green pixels was reduced, but they were not completely insensitive. From the fit in Fig. 10(b), we estimated a damage threshold of $F_{\mathrm{th}}=[14\pm 2,13\pm 2,11\pm 1,8.1\pm 0.8]\mathrm{kW}/{\mathrm{cm}}^2$ and an RBD of $2{\omega }_0=[18.5\pm 3.6,18.5\pm 3.6,18.6\pm 3.6,17.5\pm 3.8]\mu \mathrm{m}$.

3.7.

Observation, Morphology, and Threshold of CW-Laser-Induced Damage on Digital Micromirror Device

Initially, damage appeared as pixels with reduced intensity if the DMD was illuminated. Due to the fact that one micromirror was represented by 1.5 pixel in the camera image, there are always areas which are a combination of damaged mirrors and undamaged ones. Laser-induced damage to the micromirrors could cause a decrease in reflection or could destroy the tilt mechanism. Damage on the DMD started at a power density of $19.3\mathrm{kW}/{\mathrm{cm}}^2$ for an exposure time of 0.25 s. No damage was observed below a power density of $1.9\mathrm{kW}/{\mathrm{cm}}^2$. The damage on the DMD sensor is almost circular. The fit (red slope in Fig. 11) from Eq. (4) led to a damage threshold of $(21.9\pm 1.2)\mathrm{kW}/{\mathrm{cm}}^2$ and an RBD of $2{\omega }_0=(15\pm 4)\mu \mathrm{m}$.