New Delhi: Laser Science and Technology Centre (LASTEC) is the premier laser lab of DRDO involved in R&D on various laser materials, components and laser systems including High Power Lasers (HPL) for defence-applications. The main charter of the lab revolves around progressing in areas of Photonics, Electro-Optic Counter Measures (EOCM) & HPL. As a defence technology spin off LASTEC has also been engaged in the development of lasers for medical and other civil applications.
Ever since their invention in 1960, Lasers have proven to be a powerful tool with a broad range of applications from barcode scanners to nuclear fusion. The ability of lasers to generate very high radiation density at remote distances with high precision has made it ideally suited for a variety of defence applications. Laser Range Finders and Laser Target Designators are among the most extensively used defence equipments that have revolutionized the present day battlefield scenario. Precision laser guided munitions have proven to be extremely effective against adversaries under all operational conditions. Presently the focus of worldwide military laser research is shifting towards the directed use of laser energy. At moderate energy / power levels several applications have emerged in the field of EOCM for soft kill against optics and EO sensors of the adversary. Hectic development activity in area of HPL and beam control technology (BCT) are being progressed so that Directed Energy System can reach a level of maturity & can be inventoried by the Armed Forces at the earliest.
Say’s Dr. Anil Kumar, the Director of LASTEC “Most of the military laser systems such as laser designators, range finders etc., are based on solid-state lasers. The first laser was also a solid-state laser. It used a synthetic ruby rod (chromium doped aluminum oxide) with mirrors on both ends (one semitransparent) pumped with a helical xenon flash lamp surrounding the rod. Majority of modern solid state lasers now use neodymium (Nd) doped materials such as Nd: YAG (Yttrium Aluminum Garnet which is Y3Al5O12), Nd: YVO4, Nd: Glass and others. These have a much lower lasing threshold than ruby and also possess several other desirable physical and optical properties. The strongest output wavelength of neodymium-doped lasers is around 1,064 nm – near-IR and is invisible to the eye.”
Over the past two and a half decades LASTEC has acquired considerable expertise in the field of solid-state lasers. Flash lamp pumped laser sources based on different types of laser materials like Nd: YAG, Nd: Glass, Nd: Cr: GSGG, Nd: YLF, Ti: Sapphire and Alexandrite have been developed for various kinds of applications including range finding/designation, EOCM, and dual use applications like medical applications. Eye safe radiation generation has been achieved employing both KTP based optical parametric oscillator (OPO) and Er: Phosphate glass laser approaches. Development of flash lamp pumped Zig-Zag Slab Geometry lasers for high average power generation was also carried out employing indigenously developed Nd: Phosphate Glass material. LASTEC is also working on other related technologies including stable and unstable resonator design, nonlinear techniques like frequency conversion (SHG, OPO) and Optical Phase Conjugation (OPC). OPC is preferred tool which can be employed to not only auto point an HPL beam onto a distant target but also to simultaneously compensate for atmospheric aberrations in real time. LASTEC has also been actively involved in the development of state of the art diode pumped solid-state laser technology for developing highly compact, efficient and reliable laser systems. This article presents brief description of some recent achievements with special emphasis on diode pumped solid state lasers and moderate-energy solid state lasers for EOCM applications. Since the article is technically challenging, a highly respected senior scientist from LASTEC, Ms. lalita Agrawal has aided us to make this article technically sound.
1.Flash Lamp Pumped solid state lasers
LASTEC has developed basic technologies for Q-switched solid-state lasers with low energy (40-100 mJ) and peak powers of few MW suitable for range finding and designation applications. However, presently the main focus is to scale up the peak powers for variety of applications e.g. higher harmonic generation for wavelength extension and other nonlinear studies. Moderate energy lasers (1-2 J, 20 ns, 1 Hz) with second harmonic option have been developed for soft EOCM applications. A high-energy (2 kJ, 0.5 ms, single shot) laser system was also developed in collaboration with Russia for permanently damaging the front-end optics of EO systems. Recently, work on high average power of KW level disk geometry lasers has also been initiated that operate in heat capacity mode.
1.1.Dual Role EOCM laser system
A moderate energy laser system with 0.5-0.7 J output at 1064 nm / 0.1 J energy at 532 nm, has been developed and successfully tested at a range of 2.5 km against electro-optic sensors such as silicon photodiodes, CCD cameras and night vision devices. It has been suitably configured as a two men portable tripod mounted system consisting of lasers optic module, laser electronics module and power pack. A built in test system has also been developed to monitor the health of the system. Main Laser Source is based on Q-switched Nd: YAG laser Oscillator –Amplifier scheme. Second Harmonic Generation (532 nm) option including KTP crystal with wavelength selection mechanism has been included for dazzling applications. Transmitter module consists of beam delivery optics for both IR and GREEN wavelengths with desired divergences determined by suitable beam expanders.
During operation the target is acquired by gross pointing through day/night sights employing pan& tilt arrangement and depending on the application (antisensor/dazzling) suitable wavelength is selected.
Damage caused to various types of the sensors actually depends on the energy density i.e. the spot size at the target and the collecting aperture. Energy density required for human eye dazzling is about five orders less than that for PIN photodiode. Clearly all EOCM class lasers should have much higher energies than required for range-finders/ designators. Typically, the damage thresholds for the semiconductor materials are of the order of 1 J/cm2 for pulsed laser (Nd: YAG, 20 ns). In the case of pulsed lasers, it is always better to use large number of repetitive pulses to disable the sensors because the induced damages are cumulative in nature and hence the preference is for high repetition rate lasers. Since lasers in the visible region only are effective for creating flash blindness (dazzling), the choice for dazzlers is restricted to Alexandrite and frequency doubled Nd lasers. The eye safe levels for these wavelengths are of the order of 5 x 10-7 J/cm2 (20 ns – pulse duration). However exact data is not available in this particular field. In order to have flash blindness, the power level of laser appears to be two or three orders greater than this value. Moreover the laser should have a high repetition rate to achieve this effect efficiently.
Electro-optic sensors are inherently vulnerable to laser attack. Modest power laser beams are sufficient to affect the sensors in many ways. Laser can blind the sensors so that it loses its track, it can confuse the sensors to trigger the explosion of the warhead in uncontrollable manner, and it can overload sensors causing the weapon to malfunction and miss the target. Soft EOCM laser can thus be used to either permanently damage or temporary saturate or confuse the sensors. Hard EOCM laser can employed to permanently damage the front end optics of the EO devices. Similarly human eye is extremely sensitive to visible lasers, which can be employed for temporary blindness or dazzling. Such systems will render the enemy inoperative and confused during both offensive and defensive operations. In fact the EOCM lasers may offer some attractive options like low intensity warfare to fight against the threat of spreading terrorism in the world and contribute significantly to the global peace and stability. International conventions prohibit the use of lasers to damage un-aided eyes. However the convention does not cover laser damages to aided eyes.
After successful demonstration of the Dual Role EOCM system efforts have been initiated to integrate such system with fire control system of tracked vehicles like ICV & tanks including laser threat warner and a low power auxiliary laser for target acquisition.
2.Diode Pumped solid state laser technologies
As a part of DRDO PHOTONICS Programme, LASTEC has successfully developed state of the art Diode Pumped Solid State Laser (DPSSL) Technology. Solid State Lasers are traditionally pumped by noble gas (Xenon or Krypton) filled flash lamps or arc lamps. These lamps produce broad radiation from UV to infrared but only a very small portion of this wide band radiation is used by the absorbing active element. Consequently the electrical efficiency of the system is extremely poor, typically of the order of 1 %. Further, flash lamps have a life of few millions shots and require large banks of energy storage capacitors and charging power supplies. These power supplies generate few kilowatts with high peak currents. However most of these problems have been addressed effectively with the advent of high power laser diodes/arrays and use of the same as pump source for Solid State Lasers. The most attractive feature of laser diodes is their narrow spectral width fitting to the narrow absorption bands of a particular laser material (e.g. Nd: YAG), enabling efficient power, transfer and reduction of thermal loading of the crystal. The small volume resulting from smaller power supply & heat exchanger , enables compact lightweight laser systems. The low driving voltage is an additional advantage. Longer life and high reliability reduces the maintenance cost of the laser. The frequency stability and line width of DPSSL is also better than that of lamp pumped lasers due to decrease in noise because of the reduced need for cooling and more stable pump sources. Such a technology is poised to make major impact in this century.
Several critical techniques including side and end pumping configurations, pulsed and CW mode operation, laser diode driver electronics etc. have been successfully established at LASTEC leading to the development of various demonstration prototypes of DPSSL.
2.1.Passively Q-switched Diode Pumped Nd: YAG laser Prototype
A highly compact passively Q-Switched, diode pumped Nd: YAG laser system generating ~ 5 mJ @10 pps in 20 ns has been developed, which will lead to the development of ultra compact, high rep-rate laser systems for various military applications. The System combines advantages of both diode pumping and passive Q-switching techniques (using Cr:YAG saturable absorber) that result in compact and robust system with much simpler design than their conventional counterparts.
2.2.Diode Pumped Microchip Lasers
Solid-state diode pumped microchip lasers are perhaps the ultimate in miniaturization of diode pumped lasers with extremely good spatial profile, single longitudinal mode and ultra short pulse durations. These lasers are formed by application of dielectric mirror coatings directly to the two parallel surfaces of a thin slice of gain material e.g. Nd: YVO4, Nd: YAG. The short cavities can lead to inherently good mechanical stability as well as single-frequency operation. The short cavity lifetime is useful for the generation of short (sub nanosecond) pulses.
The microchip technology for generation of tens of milliwatts to hundreds of milliwatts of CW power in red, green and blue region is being widely explored all over the world. Microchip lasers find applications not only in industrial and scientific fields but they can also lead to ultra-compact lasers for defence applications.
2.2.1.Palm Top Green Laser Module
LASTEC has developed a palm top green laser module generating 50 mW power @ 532 nm in CW mode. It is based on a diode pumped Nd: YVO4 microchip laser with KTP crystal for intracavity second harmonic generation.
The module contains a 500 mW laser diode (808 nm) with integrated driver as the pump laser. A short focal length lens has been employed for focusing the beam inside the monolithic microchip laser. This laser has been also configured for short range dazzling applications by incorporating pseudorandom mode of operation.
2.2.2.High Rep Rate (70 kHz) NdYVO4 laser
Passive Q-switching of continuously pumped Nd: YVO4 has been achieved that generated ~ 16 ns wide pulses at ~ 70 KHz repetition rate. An Nd: YVO4 crystal of thickness 1mm and (3 x 3) mm cross-section was end pumped by a CW laser diode with maximum output power of 700mW @ 808nm. A Cr: YAG crystal with 90% initial transmission and an output coupler of 300mm radius of curvature (ROC) and 98% reflectivity @ 1064nm was employed. This laser can find applications in precise distance measurements etc.
2.3.Diode Pumped Eye safe lasers
A problem associated with most of the current military laser devices is that they can easily cause retinal injury in the eyes of personnel who accidentally view the laser beam even at a distance of many miles. Currently, however, rapid and significant technological innovation is taking place in the field of eye safe lasers for battlefield applications. Also during training of military operators, the use of an eye safe laser becomes highly desirable. Eye safe lasers operating around 1540 nm have the highest permissible ocular exposures as per the ANSI2.136.1-1993 standard (1J/cm2 for single pulses).
LASTEC has achieved considerable success in the field of Er:glass eye safe lasers through which direct generation of 1540 nm is possible. Active and passive Q-switching techniques have been established in both flash lamp pumped and diode pumped configurations. A flash lamp pumped passively Q-switched lab prototype has been developed generating 8 mJ, 35 ns in single shot mode. Also a diode pumped FTIR Q-switched lab prototype has been packaged generating output energy of 2 mJ with 50 ns pulse width @ 3 Hz. Packaging of a high rep rate (upto 20 Hz) laser is also in progress.
Diode pumping has considerably increased the efficiency of Q-switched Er: Glass laser from ~ 0.1% for flash lamp pumping to ~ 0.8%. It is now also possible to operate these devices at much higher repetition rates (> 20 Hz), which were earlier limited by its poor thermal conductivity. Moreover the long fluorescent lifetime (8 ms) and broad pump absorption bandwidth (910 to 1000 nm) of Yb3+ sensitized Er: Glass makes it suitable for laser diode pumping. Employing lesser number of diode arrays operating in ms regime much higher pump input energies are possible for Er: Glass in comparison to Nd: YAG laser, reducing system costs. The broad absorption bandwidth simplifies the laser diode thermal management issues, temperature control of the laser diode even over large temperature ranges is thus not required for low duty cycle lasers.
2.3.1.High Rep Rate Diode Pumped Er: Yb: Glass Eye Safe Laser
LASTEC has developed a repetitively pulsed Er: Yb: Phosphate Glass eye safe laser employing the cutting edge technology of Diode Pumping and efficient Frustrated Total Internal Reflection (FTIR) Q-switching technique. The laser is capable of generating 8 mJ in 35 5 ns @ 10 Hz with thermoelectric cooling. Such compact, lightweight high rep-rate diode pumped eye safe laser systems shall be highly beneficial for futuristic applications like range finding, lidar, optical communication etc.
2.4.Diode Pumped Nd: YAG Laser transmitter for space applications
Diode pumped Nd: YAG lasers have shown high potential in satellite missions. They have been flown on NASA missions including MOLA and GLAS and have been proposed and funded for future missions especially for remote sensing applications.
In an effort towards development of indigenous space borne laser instrumentation, LEOS (ISRO) has identified LASTEC (DRDO) as a potential partner, which can complement its expertise towards realization of successful space borne laser instruments. In this regard LASTEC is developing one unit of ‘Qualification Model’ and one unit of ‘Flight Model’ of a Diode Pumped Nd: YAG Laser transmitter for space based altimeter. The task is being completed as per a mutually agreed Joint Development Activity with LASTEC carrying out the basic laser design and development including electronics and LEOS providing support in space qualified hardware development and thermal management.
Development of space borne lasers is a highly challenging task. Laser designs must be lightweight, compact, and energy efficient. Optical mounts have to maintain precision alignment during and after launch. Out-gassing materials in space may lead to contamination of laser optics. Electronic components and optical materials must survive low pressure, thermal cycling and radiation exposures.
LASTEC has completed the design and has developed a lab prototype for design validation & performance evaluation. System consists of a diode pumped Nd: YAG laser generating 30 mJ, 10 ns laser pulses at 10 Hz. It employs an Nd: YAG laser rod of 3 x 70 mm dimension which is symmetrically pumped from three sides by laser diode arrays of total output peak power of 1800 W in 200 s @ 808 nm. A Z-shaped crossed porro prism based, electro-optically Q-switched laser resonator configuration is being employed in view of the harsh space environmental qualification requirements.
The progress in development of High Power DPSSL in India is however limited by various constraints like availability of High Power Laser Diode Arrays, Good Optical Quality Materials of large sizes etc.
Today DPSSL technology, though costly, brings together the advantages of high efficiency and compactness in comparison to conventional flash lamp pumped laser technology. For military and space-based applications where size, weight, efficiency and reliability are overriding considerations the cost becomes a secondary factor. However increased maturity in diode fabrication, assembly and testing combined with higher volume production will eventually decrease the cost per watt to a level when all solid state lasers will be diode pumped.