India ink/carbon disulfide creates laser safety device

Florencio E. Hernández, William Shensky III, Ion Cohanoschi, David J. Hagan, and Eric W. Van Stryland

Since the development of the first laser, scientists have pursued methods of protecting the human eye and optical sensors from high-power coherent light. For short optical pulses, mechanical devices are too slow and fast electro-optic devices are too expensive. Significant effort has gone into the development of new materials and devices with a high linear transmission up to a predetermined input energy, above which the nonlinear properties of the material clamp the output energy to a value below the damage threshold of the optical sensor. These nonlinear devices are called optical limiters.

Optical limiters are analogous to photochromatic sunglasses, in which sunlight changes the optical properties that can be reversibly modified by light. To block a short pulse of light, a nonlinear optical material must have a much faster response and larger induced absorption than photochromatic sunglasses. These optical limiters need to meet specific requirements such as high linear transmittance, chemical stability, response to short and long temporal pulses, wavelength independence, and a high resistance to permanent laser-induced damage.¹

For light entering the human eye, the encircled energy (the energy passing through an aperture of ~1.5 mrad placed at the focal point of the collecting lens) must be kept below the energy at which catastrophic damage of the retina occurs (1µJ).² The difficulty of protecting the retina is that the lens increases the irradiance by nearly 10⁵ from the irradiance at the iris. For "eye-safe" wavelengths, light is absorbed by the cornea and lens so that it never reaches the retina. Dangerous pulse energies for the cornea are orders of magnitude higher. Therefore, visible and near-infrared wavelengths are potentially the most damaging to the eye.

Optical limiting materials/mechanisms
Many different types of materials and nonlinear mechanisms have been studied for this specific application during the past 10 years. It has been demonstrated, for example, that two-photon absorbers make good optical limiters because they have negligible linear absorption. As irradiance increases, two-photon absorption causes the material to become a strong absorber and, at the same time, limits transmittance for relatively low energies. Two-photon absorption processes are irradiance dependent. Therefore, they work well for picosecond pulses but not for longer nanosecond pulses (the ratio of irradiance to fluence or energy per unit area decreases, thus decreasing the nonlinear effect).

Within the nanosecond temporal regime, however, it has been shown that molecules possessing an excited-state absorption cross section greater than that of the ground state are more appropriate because they are fluence dependent. These types of molecules are known as reverse-saturable absorbers. Even though reverse-saturable absorber materials display good optical limiting in the nanosecond regime, they still have the limitation of a low damage threshold using a single nonlinear element in a single lens-focusing geometry—the geometry commonly studied in the past. Thus, the dynamic range of the device is usually low. This important parameter, the dynamic range or figure of merit (FOM), is defined as the linear transmittance divided by the minimum transmittance at high energy. While not enough to measure the absolute performance of an optical limiter, the FOM indicates how usable a new nonlinear material or experimental design will be for this kind of application.

Improving performance
It has previously been demonstrated that the FOM of an optical limiter can be extended to approximately 500 by using a single-focus geometry with three separate reverse-saturable absorber elements placed along the focusing beam (tandem configuration)—one order of magnitude greater than that of the standard single-element configuration. This result is encouraging, although the maximum output energy is still above the acceptable value for eye protection and the damage threshold is low.

Not long ago, researchers at the University of Central Florida's School of Optics (Orlando, FL) developed an f/5 cascaded-focus optical geometry that consists of two nonlinear elements with two foci to improve the performance of traditional optical limiters. One 2-cm-thick cell contains pure carbon disulfide (CS₂) at the first focus, while the other element is a 100-µm-thick cell with a reverse-saturable absorber solution of lead phthalocyanine dissolved in chloroform (PbPc/ChCl₃) at the second focal plane (see Fig. 1). Using nanosecond pulses and working at a wavelength of 532 nm, the cascaded-focus optical limiter clamped the output encircled energy below 1 µJ for inputs up to 58 mJ without any damage (see Fig. 2). This provides a FOM >30,000, which is the highest-dynamic-range optical limiter ever reported.

The high performance of this particular optical limiter is based on the combination of two nonlinear materials (with different nonlinearities) in a cascaded-focus geometry. Although this geometry consists of a more complex design, it adds versatility and results in the upright image needed for viewing. A CS₂ cell located in the first focal plane protects the second cell against damage for high input energies. The second cell shields the optical sensor for relatively low inputs.

The operation of this device to extend the dynamic range is based on temporary "optical damage" that occurs once the critical power for self-focusing is reached in liquid CS₂. This results in strong scattering created by laser-induced breakdown and plasma generation limiting the transmittance through the system. The strong self-focusing in CS₂ is caused by the Kerr effect and electrostriction. The electrostriction contribution is comparable to that of the Kerr effect for these experimental parameters and shifts the critical power to energies below the damage threshold of the second element—protecting it from damage. Since CS₂ has negligible linear absorption within the visible and near-infrared, it is possible to maintain a high linear transmittance through this system.

Broadband limiter
In recent years, scientists developed high-power coherent sources that can emit light from the ultraviolet to the infrared. It is important, therefore, not just to concentrate on limiting over a narrow spectral range, but on devices that are broadband limiters as well. The latter is a challenging task because reverse-saturable absorbers are wavelength-dependent, and the previous combination of nonlinear materials works only within a relatively narrow band of wavelengths in the visible centered in the green. Thus, it was necessary to find a broadband limiter to be inserted at the second focus of the cascaded-focus optical limiter.

Among all the reported materials for optical limiting, one of the best broadband materials to date is carbon-black particles in suspension (CBS), which is basically just dilute India ink. The dominant nonlinear mechanism in CBS is strong scattering caused by laser-induced plasma formation and bubble generation. The particles strongly absorb light, which causes them to heat up and eventually explode.

The advantage of using CBS over reverse-saturable absorbers is that this process is nearly wavelength independent and is just as effective as an absorber. The apparent drawback to using CBS is that when it is irradiated with multiple pulses, the carbon particles are depleted and the transmittance rises. This increase in transmission may be seen at laser repetition rates of only a few hertz in common solvents such as water and ethanol. At lower repetition rates, the increase is not seen because diffusion of the suspension replenishes the CBS.

The key to overcoming the apparent shortcoming in CBS was realizing that this effect is viscosity dependent, as carbon particles in the depleted region can be replaced by diffusion between consecutive pulses. The rise in transmittance shifts to higher energies as the viscosity of the solvent decreases. In the low-viscosity solvent CS₂, the limiting is unaffected by the repetition rate for rates up to 10 Hz. After establishing this, the cascaded-focus optical limiter was modified by replacing the reverse-saturable absorber solution with CBS in CS₂.

Experimental results
Experimental results for the cascaded-focus optical limiter using CBS at different wavelengths within the visible spectrum show a strong scattering caused by plasma and bubble generation in both cells that limit the total output encircled energy below 1 µJ for inputs up to 1 mJ—the maximum output energy of the optical parametric oscillator system used in the experiment (see Fig. 3). These experiments were performed using a Continuum Sunlite-EX tunable optical parametric oscillator system, pumped by a frequency-tripled, Q-switched, Nd:YAG laser, producing 5-ns pulses at a 10-Hz repetition rate. The measured low-irradiance transmittance for the whole system with a 5-mm CBS cell was approximately 25%. Although the linear transmittance was reduced, a broadband response was gained.

To measure the dynamic range of the limiter using CBS at the second focus, the experiment was repeated at a wavelength of 532 nm for higher energies (E_in = 62 mJ). The measured dynamic range at this wavelength was very high (FOM >20,000), while the encircled output energy remained clamped below 1 µJ for the entire range of inputs. It is important to highlight that no damage to the device has been observed.

REFERENCES

1. R. C. Hollins, SPIE 3282, 2 (1998).
2. V. Grolier-Mazza, Nonlinear Opt. 21, 73 (1999).
3. F. E. Hernández et al., Opt. Lett. 25, 1180 (2000).

FLORENCIO E. HERNÁNDEZ, WILLIAM SHENSKY III, ION COHANOSCHI, DAVID J. HAGAN, and ERIC W. VAN STRYLAND are all members of the University of Central Florida's School of Optics, 4000 Central Florida Boulevard, Orlando, FL 32816-2700; e-mail: [email protected].