Manufacturers have referred to lasers with operating wavelengths longer than 1400 nm (mid to far infrared) as eyesafe. Wavelengths in this region are absorbed in anterior portions of the eye (mainly cornea) and therefore never reach the retina. This is in contrast to the eye- hazardous portion of the optical spectrum of 400 - 1400 nm (visible and near infrared) where the anterior portions of the eye have high transmittance and refractive power. Irradiance levels are typically 5 orders of magnitude greater at the retina than at the cornea for visible and NIR wavelengths. Although wavelengths longer than 1400 nm do not interact with the retina, they can interact with the skin or cornea and cause a thermal injury. Specifiers of laser equipment have required that systems be eyesafe at a particular distance with or without optics. By this they mean that exposure at that distance for the naked eye or optically aided viewing through a specified power of optics should not exceed exposure limits. Some wavelengths in the visible and near IR are more 'eyesafe' than others because the safety factor between the level that would actually produce an injury (ED-50 value) and the exposure limit is greater. For example, the ED-50 value for a single Q-switched laser pulse for the ruby laser wavelength of 694.3 nm is 11.2 (mu) J into the eye and the Accessible Emission Limit (AEL) is 0.19 (mu) J, a factor of 59 between ED-50 and the AEL. The ratio for 1064 nm for a single Q- switched pulse is 99 (mu) J/1.9 (mu) J or 52. So, in a sense, exposure to 694 laser radiation at the AEL, is slightly more 'eyesafe' than exposure to 1064 nm radiation at the AEL. To the laser safety specialist, however, eyesafe, means only one thing, Class 1. Class 1 is a laser hazard classification that means the laser does not exceed specified limits for a reasonable, worst-case combination of distance, exposure duration, and collecting aperture diameter during its intended use. Determining whether a laser is Class 1 is usually a two-step process. The first step is determination of the AEL and the second is determining whether or not the laser in question exceeds this AEL. Exceeding the AEL is determined by calculation or measurement. If the AEL is exceeded under prescribed parameters then the laser is not Class 1. Differences in laser safety standards in both of these classification steps have caused confusion. Differences in exposure limits chosen by laser safety specialists based upon the same criteria have also caused confusion.
High performance receivers allow lower-power, smaller lasers and assist in size reduction of a new generation of eyesafe laser rangefinders. Aspects of receiver design and trade-offs are covered to assist systems and application engineers.
Planar InGaAs/InP avalanche photodiodes (APD's) for use in eye-safe laser rangefinding applications in the 1100 to 1700 nm spectral range are discussed. The devices have diameters of 85 and 200 micrometers , having capacitances of 0.7 and 2 pF, respectively. For the diodes operating at a responsivity of 10 A/W at 1540 nm, for which the gain is approximately 10, measured noise currents at room temperature are 0.5 and 0.75 pA/(root)Hz, respectively, and frequency response is greater than 400 MHz. Results are also reported for a back-entry version of the 200 micrometers diameter APD fitted with a 600 micrometers diameter high index ball lens for enlarging the effective sensitive diameter. Quantum efficiency for the unit is found to be about 65%, and good response uniformity is achieved over a diameter greater than 500 micrometers , using an f/1.8 optical system.
The structure and device properties of indium gallium arsenide (InGaAs) pin photodiodes and avalanche photodiodes (APDs) are described. Quantum efficiencies above 85% at 1.54 micrometers , dark current densities near 1 (mu) A/cm2 (-5 V, 300 K) and 3 mm diameter shunt resistances (10 mV, 300 K) above 10 megohms have been observed. Avalanche gains above 20 have been measured with multiplied primary dark currents below 7 nA. Extended wavelength InxGa1-xAs (.53 < x < .80) pin detectors are also described with 70% quantum efficiency and room temperature RoA products above 2000 ohm -cm2 at 1.8 micrometers , 900 ohm -cm2 at 2.1 micrometers and 15 ohm -cm2 at 2.6 micrometers .
All laser eye protection (LEP) should be tested to insure that it meets certain performance criteria. There are no American standard test methodologies other than military standards for LEP to meet performance criteria. Depending on the type of eyewear, the actual protection afforded during laser exposure may be different from the protection measured by standard methods. Specific user application environments may require additional tests for laser eyewear such as solar, temperature, and humidity stability. Optical quality testing methodologies including optical distortion, refractive power, prismatic deviation, haze, and chromaticity coordinate measurements are discussed along with the importance of user acceptance in optical quality, field-of-view, weight, comfort, corrective lens compatibility, and style.
The search for simple and effective methods of erbium glass lasers (lasing wavelength 1.54 microns) Q-switching is a task of present interest. It is interesting because this wavelength radiation is relatively eyesafe and may be used in medicine, lidars, and fiberoptics. Nowadays, existing active methods of Q switching are rather inconvenient for practical use (rotating prism) or complex and not too efficient (electrooptic cell). As for passive shutters based on color centers in crystals, or organic dyes, nowadays they are only under development, and Q—switching is obtained in them only as a small effect. In this report, two new simple methods of passive Q-switching of erbium glass lasers are suggested, and the main properties of obtained giant pulses are investigated.
Erbium population dynamics in ytterbium sensitized phosphate glass is studied by measuring transient changes in laser probe transmission during flashlamp pumping. The influence of energy loss channels such as excitation cumulation and nonlinear fluorescence quenching on the flashlamp pumping efficiency is observed to be relatively small. The decrease in the pumping efficiency at high input energies can be attributed mostly to the shift in the flashlamp output radiation spectrum.
We report on new eyesafe laser hardware developed at Hughes Aircraft Company. We also report on the development of a compact 20 Hz pulse repetition frequency (PRF) backward Raman configuration and the investigation of deuterium as an alternative to methane for high PRF eyesafe Raman lasers.
An eyesafe laser rangefinder technology based on repetitively pulsed diode lasers in the wavelength range between 800 and 900 nm has been developed. Single pulse energies and pulse train mean power have been calculated to be in accordance with eye safety regulations and to provide an optimum signal-to-noise ratio. Applications for this technology are short and long distance range finders which work on non cooperative targets.
The Sight Integrated Ranging Equipment (SIRE) incorporates an eyesafe laser rangefinder into the M-36 periscope used in tactical armored vehicles, such as the Commando Stingray light tank. The SIRE unit provides crucial range data simultaneously to the gunner and fire control computer. This capability greatly reduces 'time-to-fire', improves first-round hit probability, and increases the overall effectiveness of the vehicle under actual and simulated battlefield conditions. The SIRE can provide target range up to 10-km, with an accuracy of 10-meters. The key advantage of the SIRE over similar laser rangefinder systems is that it uses erbium:glass as the active lasing medium. With a nominal output wavelength of 1.54-microns, the SIRE can produce sufficient peak power to penetrate long atmospheric paths (even in the presence of obscurants), while remaining completely eyesafe under all operating conditions. The SIRE is the first eyesafe vehicle-based system to combine this level of accuracy, maximum range capability, and fire control interface. It simultaneously improves the accuracy and confidence of the operator, and eliminates the ocular hazard issues typically encountered with laser rangefinder devices.
Revived interest in mid-IR lasers can be attributed to better materials, medical applications, and their eyesafety. Twenty years ago, these 2 to 3 micron lasers were limited to research labs because of the necessity of cryogenic cooling. Recent advances have made room temperature operation with manageable thresholds available for Co:MgF2, Tm, Ho, and Er covering from 1.7 microns to 2.5 microns. The interest in medical applications is related to the high absorption for tissue at these wavelengths and the flexible delivery by low fiber optics. The eyesafety issue makes commercial uses more attractive. Other interesting applications where eyesafety is critical include: rangefinders, remote sensing, wind shear detection and lidar.
Many applications exist for eye safe laser operating at high repetition rates. This paper will discuss the operation of Q-switched Er:glass lasers at high repetition rates with peak powers in the megawatt range.
Training is a process of imparting a particular set of skills to a target group either by having them perform an actual task until proficiency is gained or by performing a similar task until confidence of proficiency is attained. Doing an actual task may be preferred but many factors may dictate that this objective is not feasible. The armed services and civilian law enforcement groups must train to use their weapons but often weapon characteristics, expense and the availability of appropriate facilities dictate that some sort of simulation be employed. Eyesafe laser are playing a major role in this sort of simulation. Present uses include their employment as replacements for non-eyesafe lasers in determining the distance to a target, designating a target for laser energy seeking munitions and to signal the arrival of a munition at a target is a benign manner compared to what the replicated munition would do were it used instead.
A laser training system entitled Shoot Through Obscuration MILES (STOM) is being developed to operate with Forward Looking InfraRed (FLIR) systems during battlefield exercises where visibility is impaired. The STOM system is capable of ranges in excess of 6 km and can penetrate battlefield obscurants such as fog-oil, smoke, dust, and rain.
Eyesafe military laser range finder systems that incorporate carbon dioxide lasers operating at 10.59 microns have been successfully developed and are currently in production for both the U.S. and foreign military services. The development of carbon dioxide laser rangefinders for Fire Control applications has provided high performance, eyesafe capability to both heavy combat vehicles and air defense platforms. The distinct wavelength compatibility provided by a long wavelength laser in conjunction with long wavelength Forward Looking Infrared (FLIR) detection systems positions the CO2 laser range finder system as unique in the ability to provide ranging capability to FLIR recognizable targets and eye safety. First order modeling of expected performance versus system design parameters has provided a basis for understanding the key performance factors and their relationship to necessary design trade-offs. The successful implementation of the laser range finder design required the development of an array of CO2 laser system components that provide both the transmitted laser pulse and the ability to detect the target reflected return. A modular design approach to the CO2 laser system components has led to several successful programs that incorporate identical key technologies thereby reducing the overall cost of all CO2 laser range finder programs.
An eyesafe source (1.61 micrometers ) with 1.1% wallplug efficiency, is demonstrated using a Nd:YAG pumped KTP optical parametric oscillator with peak-power conversion efficiencies of 70%. Joule-level scaling, kHz repetition-rates, and ns pulselengths are now accessible using this technology.
A high precision multi-target, short-range laser range-finder using a semiconductor laser diode is described. The laser diode is intensity modulated with a time-dependent frequency voltage signal. The return bundle is detected by a semiconductor photo diode and mixed with an undelayed fraction of the time-dependent frequency signal. This produces sum and difference frequencies, of which the difference frequencies are filtered out and are analyzed for individual components by a fast fourier transform processor. Each individual frequency component represents a specific distance to a target. The optical transmit and receive bundles are coaxial and can be scanned by a mirror scanner up to 100 Hz in elevation and 10 Hz in azimuth over a 30 degree(s) by 30 degree(s) field of view. The estimated accuracy in distance is 10 cm with a 1 ms 1.5 Ghz chirp and 36 mW of optical power at a wavelength of 1310 nm, over distances ranging from 1 to 50 m. The fast fourier transform processor estimates the up to 512 individual frequency components in less than 1 ms. The non-linear time-dependent frequency behavior of the voltage controlled oscillator is compensated by an optical feedback path. This contributes greatly to the accuracy of measured distances. This study has been carried out in the framework of IEPG activities on autonomous guided vehicles.
Two types of high repetition rate eyesafe rangefinders are considered. The
first is an Nd:YAG laser rangefinder that operates at 1.54 microns. The second is a
MIL-qualified laser diode designed for a missile.
It is proposed to construct holographic devices which act as optical elements to modify the path of high spatially coherent radiation but not affect incoherent radiation. The device be composed of a laser-read holographic element in association with a band-pass spatial filter. The holographic filters will be wavelength independent to allow possible action as agile filters for laser safety protection while maintaining sufficient transmission for visual performance.