SOURCE: Indrajit Majumdar / FOR MY TAKE / IDRW.ORG
It’s been more than 30 years now from when India truly started perusing its DEW dreams especially in Laser technology. It was Laser Science and Technology Centre (LASTEC), the key institution on whose shoulder it was putting its trust for its dreams to become real. Since then LASTEC developed and tested several types and classes of Laser sources, Lasing mediums, associated technologies for each types and classes, several types of laser applications etc. etc.
LAB in the Centre Stage:
Yes the LASTEC it was, the central lab responsible for all the actions on that field. And there are reasons that why LASTEC and why not others. The question is valid because peoples may think that when the LASTEC stands for Laser Science and Technology Centre, it’s obvious that it will be given the work because it was meant for the type of work.
But unfortunately most peoples don’t know that LASTEC was not LASTEC when it was given the responsibilities of making India sound in the field of lasers and other DEWs. SO, the question now seems valid.
The Story of LASTEC:
LASTEC is not any ordinary institution; it’s the oldest lab in today’s DRDO (Est. 1958) establishments. In fact it’s the reincarnation of the old Defence Science Laboratory (DSL) of 1950s which initially operated from the National Physical Laboratory (NPL) building complex. That the DSL which gave birth to as many as 15+ most critical full-fledged labs in the current possession of DRDO, including:
Defence Research & Development Laboratory (DRDL)
Solid State Physics Laboratory (SSPL)
Institute of Nuclear Medicine & Allied Sciences (INMAS)
Food Research Laboratory (FRL)
Institute for Systems Studies & Analyses (ISSA)
Defence Scientific Information & Documentation Centre (DESIDOC)
Centre for Fire, Explosives & Environment Safety (CFEES)
Scientific Analyses Group (SAG)
Institute of Technology Management (ITM).
To start with, in 1950s, the research activities of the laboratory were mainly confined to operational research and ballistics. Electronics and communications, explosives, physiology, nuclear medicine research and food technology were added to its areas of research and study in 1960s. In 1970s, the laboratory consolidated its R&D activities towards more specific and application oriented areas, such as liquid fuel technology, spectroscopy, crystallography, and so on. The laboratory contributed significantly in missile programme in the areas of G-fuel and UDMH (for long and short range guided missiles), trajectory modelling and Joule-Thomson mini cooler and IR dome material, polyurethane for potting of electronic circuits, microphone grid (for locating gun position by sound ranging methods), air ventilated suits, etc.
In 1982, it was given a new charter of duties with its major thrust on lasers. So to accommodate with the new duties the DSL moved to its new technical building in Metcalfe House complex, Delhi and was renamed as Defence Science Centre (DScC). The Centre was made responsible for the development of lasers for directed energy applications as one of its major missions. This was the initiation of India’s DEW dreams.
Here comes the LASTEC:
In 1999, in view of the R&D thrust shifting to development of lasers, Optoelectronics and related technologies, the laboratory was rechristened as Laser Science and technology Centre (LASTEC).
LASTEC has since established itself as a centre of excellence for the development of high power laser sources and related technologies, electro-optic countermeasure and battlefield optoelectronic equipment.
LASTEC in the Work:
With the new responsibilities on shoulder LASTEC initiated the work. In the process they developed several types of laser source technologies for Directed Energy Weapon (DEW), dazzling and imaging applications. Standalone sensor systems using different laser sources for applications like detection and location of optical targets and detection and identification of chemical, biological and explosive materials, electro-optic countermeasure systems and laser materials to name a few. Scientific principles and techniques such as Raman scattering and its variants, laser photo-acoustics, laser induced fluorescence, differential absorption, etc. have been aptly applied to develop a number of equipment for detection and identification of various chemical, biological and explosives warfare agents in field conditions. These equipments are at various stages of evaluation and have tremendous application in low intensity conflict operations. A number of sub-system level technologies for building the most modern state of the art laser systems for military applications have also been successfully developed. Expertise in associated technologies like beam pointing and tracking, embedded system design and thermal management has been achieved.
FACET, a state of the art facility for test and evaluation of laser systems has been established at Ramgarh, Chandigarh. LASTEC is committed to provide world class laser sources and systems, using state of the art technologies and complying to the world standards.
Over the years, LASTEC has acquired the expertise in designing, testing and evaluation of different types of laser sources and systems. Gas Dynamic Laser (GDL) and Chemical Oxygen and Iodine Laser (COIL) Sources of the order of tens to hundreds of kilowatts for DEW application have been successfully developed and demonstrated. Recently, single mode kW class Fiber Laser Source was realised in collaboration with foreign experts making India only the 6th (Known) country to possess the requisite technological knowhow. Efforts are channelized in scaling the power levels of these laser sources.
The systems being developed are contemporary to those developed by the world leading military laser manufacturers and are appropriate to the Indian conditions.
Equipment Level Laser Systems Development:
With the help of the researches done by the LASTEC on the core technologies of different types of lasers and their sources DRDO developed various product level laser based equipments which can be used for various applications including battlefield and policing purposes. Below are some of the systems.
Laser Ordinance Disposal System (LOrDS):
LOrDS is basically a system for detonating surface laid unexploded explosives (i.e. Mines, IED etc.) from a safe standoff distance using laser energy.
It focuses laser energy on the casing of the munitions thereby heating it until the explosive fillers ignites and starts burning. This happens in seconds. The combustion of the explosive fillers of the munitions leads to low level detonation or deflagration of the munitions.
This brings several advantages, safe standoff distance, ultra-high precision, fast disposal and reduced collateral damage among these.
The system consists of a TATA made vehicle called LSV (Laser Standoff Vehicle). A motorized gimballed steerable laser optics module. This module have a main kw class fiber laser (i.e. the business end) at 1070 nm wave length, a high accuracy laser range finder unit at 532 nm, a variable zoom CCD camera, integrated and bore sighted with laser unit is used for target sighting, A visible laser beam is also provided for aiming the hit point on a target. The operation of the module is controlled by a single operator through a command control console provided in front of co-driver seat.
The whole thing is assembled in a casing and much looks like a big dual lens projector and is the laser optics module we are talking about. The laser optics module is mounted on a two axis servo pedestal controlled Beam Control System (BCS) module which is controlled by a Fire Control System (FCS). All the heat generated by the main laser and the LRF system is controlled by the Thermal Management System (TMS). Prime Power Unit (PPU) is responsible for providing and managing all the required power. The system can also be remotely controlled by a Remote Control Unit (RCU).
The experimental trials on LORDS were jointly carried out with ARDE and TBRL for evaluating system potential and capabilities at Ramgarh Range, Chandigarh. The trials were conducted on low and medium calibre ordnances from stand-off ranges of 150 m and 250 m. About 13 different types of munitions had been tested and more than 45 disposals had been conducted till date.
Optical Target Locater (OTL):
In the modern world today OTL has emerged as a crucial tool for covert laser surveillance and even for sanitation of a specific area.
LASTEC has developed an Optical Target Locater (OTL 300) for the detection of a passive or active optical threat for operational ranges of snipers. The system functions on Cat’s eye effect. Any optical system when illuminated by a laser beam returns some back scattered energy. This retro-reflected energy provides the location of the optical target against the background. The system provides an important tool for detection of any active or passive surveillance device using the retro-reflected signal from their front end optics. The threat can be in the form of a sniper equipped with a day sight or a night vision device, or any other optical/electro optical surveillance device, viz., binoculars, surveillance cameras, laser range finders, designators, etc.
In OTL 300, both the target and the background are clearly visible for estimation of target location. Since target detection through the OTL is based on achieving high contrast between the retro-reflection from the front end optics of the optical threat and the background scene, the system parameters have been optimised to achieve high contrast between the retro-reflected signal and the background. The system is robust, lightweight, compact and user friendly for efficient operation. Equipment with capabilities of higher ranges, network ready operations are also being developed.
In recent years, the laser dazzler systems have evolved as highly potent threat deterrent for area denial. The conventional means of dispersing or controlling an agitated crowd can cause serious and lethal injuries. Laser dazzler which transmits a laser beam in visible spectrum temporarily impairs or disorients the aggressors in a completely non-lethal manner. Laser dazzlers are non-lethal weapons specifically designed for applications where subject vision impairment must be achieved at specified distance in all ambient conditions including clear sunny day, twilight and night. Laser dazzlers can be configured for variety of configurations from hand held to weapon mountable to vehicle mountable. LASTEC has developed variants of laser dazzler for diverse applications. Laser dazzlers from a range of a few tens of meters to tens of kilometres have been developed. Long range laser dazzler (Helios-AD) is a concept demonstrator prototype for dazzling at ranges of tens of kilometres. The system employs EO payloads mounted on two axis gimbal for target detection, identification and tracking for ranges in excess of 10 km. The system launches a preliminary warning to the threat using low power modulating lasers. If the threat proceed in the same direction despite warning, highly intense yet safe dazzling green laser hails the threat platform forcing it to manoeuvre the platform in alternate direction. The system is integrated on mobile platform (Stallion).
Some other works on laser based equipments:
LASTEC has developed several other kinds of devices e.g. devices for sensing of Chemical, Biological and Explosive Agents. For this they developed different types of LIDER (Light Detection And Ranging) techniques for example CO2 lasers (emitting at ? = 9-11 µm) based Differential Absorption LIDAR (DIAL) and Ultra Violet Laser Induced Fluorescence (UV-LIF) for biological agent detection.
LASTEC has designed and assembled two differential absorption LIDAR (DIAL) sensors. First one uses OPO technique based laser operational in 3-3.5 ?m and the second one uses a tuneable TEA CO2 laser based system operational in 9-11?m IR band.
Ultra Violet Laser Induced Fluorescence (UV-LIF) is a fast emerging technique for the development of such systems for stand-off detection. Trolley Mounted UV- LIF LIDARUV-LIF LIDAR technology for the detection of biological agents using CCD optical fibre spectrometer has been developed at LASTEC. Multi Anode Photo-Multiplier Tube (MAPMT) spectrometer based and intensified CCD based UV LIDAR sensor is under development for stand-off range.
Explosive Agent Detection is another field of work being done in LASTEC.
Currently, a number of technologies exist that can be used to screen people, luggage and vehicles but internally LASTEC’s standards are pretty much high. Stand-off detection is thought to be the most desirable proposition to meet any kind of situation including pre-empting of the threats in advance by covertly interrogating them from remote and safe distances. The best hope for the stand-off detection is the analysis of particulates absorbed on surfaces by using a laser to cause these explosive particulates to emit characteristic (spectral) radiation. Though there are many laser techniques available, however only a few of them have the potential for stand-off detection. LASTEC has experimented with Raman Scattering for this application. It includes Surface Enhanced Raman Scattering (SERS); Resonance Raman Scattering (RRS); and Coherent Anti-Stokes Raman Scattering (CARS). Laser Induced Fluorescence (LIF), Laser Induced Breakdown Spectroscopy (LIBS), Photo Acoustic Spectroscopy (PAS), Open-path Fourier Transform Infrared Spectroscopy (FTIR), and Multi-spectral Infrared Imaging are some other important techniques for the stand-off detection of explosives. Each of these techniques has certain advantages and limitations.
Three prototypes based on Raman and pre-resonant Raman spectroscopy has been developed at LASTEC.
Pre-emptor which is successfully tested for detection and identification from stand-off distance of 30 cm to 5 m.
Second type of equipment is a trolley mounted Vishleshak-I system which is capable of detecting explosives from variable distances of 2m up to 30 m. The system is based on time gated pre-resonant Raman detection technique. Sensitivity of the system is 100 ppm from a distance of 5 m.
The third prototype is a tripod mounted system Vishleshak-II which is developed for detection range from 1 to 50 m using Pre-resonant Raman scattering. The system uses a pulsed UV laser for illumination of the target material. Collecting optics focuses the scattered light on CCD coupled spectrometer.
LASTEC has also developed tuneable quantum cascade laser based Quartz Enhanced Laser Photo Acoustic Sensor (QE-LPAS). Its highly sensitive spectroscopic technique for stand-off detection of explosive material and are easy to use in field conditions in the presence of sunlight and attain detection range up to 100 m for traces. Patent titled “Stand-off Detection of Chemical, Biological and Explosive Detection using Quantum Cascade Laser Photo Acoustic Spectroscopy” has also been filed.
Another variant of LPAS, i.e. Microphone based LPAS system is currently under development for stand-off trace detection of explosive material.
Several other examples of their work are, Air Borne Laser Spot Detector, Land Based Laser Spot Detector, Laser Warning & Countermeasure System, Laser Threat Detection and Decoy System, Laser Warning System, Decoy Laser, IR Guided Missile Tester, Griffin LGB Kit Tester, Laser Cross Section Measurement and Imaging etc. etc.
Weapon Grade Laser Systems Development:
High Power Laser Sources based Directed Energy Weapons (HPL-DEW) have emerged as one of the most revolutionary classes of weapons in the 21st century. Ground based deployment of laser based DEW for air defence against rockets and missiles has already been established. High power airborne laser has also been tested against ICBM up to a range of 150 km. Key technologies for space based lasers with very promising anti-missile and anti-satellite capabilities have also been proven. Global influence of this new generation of high power laser sources based DEW can potentially rewrite the ground rules for the future warfare doctrines and geo-political equilibrium in the 21st century. The technology has progressed significantly in the past decade and many nations are currently engaged in the rapid development of high power laser based DEW Systems. In India, DRDO is vigorously pursuing development work for both tactical and strategic DEW application.
Gas Dynamic Laser (GDL), The Aditya Project:
Aditya project was basically an experimental test bed to seed the critical DEW technologies. Its a 100 kw class Gas Dynamic Laser which can cause the stipulated damages at 0.8 km and 2.5 km distance using a 0.7m aperture telescope. The Gas Dynamic High Power laser based Directed Energy System can be broadly divided into two major subsystems: Laser Power Source and Beam Delivery System. The laser source consists of system integrated on different transportable platform, viz., Laser Generation System Vehicle (LGV) and Air Supply System Vehicle (ASV). The system has been realized, integrated, and tested. Laser power of the order of 100 kW has been achieved. Damage potential of the laser has been tested up to 800 m distance.
Laser Generation Vehicle (LGV):
LGV has been developed to generate a high energy beam. The input energy is pumped into the system through combustion of large amount of air and toluene. The system mainly consists of laser tunnel, toluene storage, gasoline storage and pressurisation and feed system, parts of combustion, ignition, aerodynamic window, cavity bleed, fuel purging, pneumatic air supply lines, control panel, etc. The laser tunnel consists of a combustor, a nozzle bank with cavity, resonator, diffuser and TCD. These systems are mounted on a low bedded transportable trailer having suitable canopy. The LGV is dependent on ASV for oxidiser air supply and command control vehicle for operation commands for generation of high energy beam.
The associated technologies developed are:
1.Toluene – Air Combustor System for Generation of Lasing Gas Mixture @ 40 kg/s
2.Supersonic Multi Bank Contoured Nozzle
3.Resonator Cavity along with Mirror Protection System
4.Direct Discharge Supersonic Diffuser
5.Vortex Aerodynamic Window
6.Vibration Isolated Resonator
7.Data Acquisition & Control System
Air Supply System Vehicle (ASV):
Air Supply System Vehicle Air storage and supply system delivers the compressed air at different pressures and different flow rates to various systems. It consists of high pressure air generation, storage and supply system. In this system large quantity of air is stored which is required at the time of laser test firings. Air is required in many sub-systems like main air to combustor, air to igniters, air to aero-dynamic window, air for pneumo-pusher, cavity bleed, air for mirror protection and aerodynamic window shutters, air for Remotely Operated Valves (ROV), air for purging of main fuel and igniter fuel lines, intermediate cooling, etc. This vehicle is capable of supplying up to 50 kg/s mass flow rate of regulated air for combustion, aero-window and other miscellaneous purposes.
Command Control System:
The command control vehicle is an ISO type air conditioned EMI protected shelter to control the operation of laser source and beam delivery on the target from a safe distance. This is achieved through two control consoles for the GDL source and beam director mounted into the command control vehicle. The GDL control console is capable of laser testing the operation of various valves and conducting laser firing. It is very critical to acquire data from the field sensors on LGV, ASV, BDV, and controlling all the subsystems for remote firing control. The safety interlocks have been implemented based on pressure in igniter and combustor for avoiding any pulse which can damage the system. The beam director console is capable of independently acquiring the information of the target and the delivery of laser beam on to it.
Power Supply System Vehicle:
The power supply system is mounted on a DG set vehicle. It houses three DG sets. Two numbers of higher power DG sets are used to run the compressor placed on air storage and supply trailer. The third lower capacity DG set caters to the charging needs of various UPSs placed in other trailers/ vehicles and other electrical requirements of the DEW system.
Beam Delivery System:
The beam from laser source is coupled to the beam delivery system. The focusing telescope is mounted on a gimbal which should be agile enough to keep the beam focused on the same spot. The gimbal has to work in closed loop with video tracking system. The main objective of the Beam Directing System (BDS) of Project ADITYA is the auto focusing of the high power laser beam onto a distant moving target in the required operating range. The BDS consists of mainly two assemblies, one is beam transport system and other is stabilised gimbal platform assembly. The stabilised gimbal platform along with beam directing electronic system is responsible for controlling the overall functionality of the system.
* The beam directing telescope is one of the most critical subsystems of the laser beam director which is responsible for coupling the laser power from the source to the target and for precise pointing/ focusing of laser beam to achieve the required power density on to the area of interest on the moving target at the operating range and for the entire duration of the laser radiation.
* Target acquisition and video tracking is used to detect, identify and track the moving airborne targets, so that high power laser beam can be fired on the vulnerable spot of the target. Once the target is acquired, detected and identified, the lock on and tracking gets initiated. As the target comes closer to the beam director, then area of interest, i.e. vulnerable spot on the target is locked and tracked.
Test and Trials of the System:
The beam was focused on the far field targets with the help of Basic Beam Director (BBD) and gimballed telescope mounted on the Beam Delivery System Vehicle (BDV). Damage potential on various types of targets (stacks of mild steel plates, perspex, GFRP, optical glass, etc.) has been established.
Chemical Oxygen Iodine Laser (COIL) Project:
Chemical Oxygen Iodine Laser (COIL) systems are amongst the most important candidates in the class of High Energy Laser (HEL) systems for strategic applications. COIL systems provide several unique advantages such as good atmospheric transmission characteristics and short wavelength (1.315 mm) which facilitates reasonable spot size at long ranges with manageable optics. COIL is a low pressure system (cavity pressure ~ 3-5 torr) and offers extremely good beam quality, exhibits potential for greatly reduced production costs due to the use of plastic parts throughout the device. COIL is capable of being scaled up to MW class power levels in a single aperture output and has been tested against cruise missiles. COIL requires singlet oxygen for lasing. Singlet oxygen is generated by reaction of basic hydrogen peroxide liquid and chlorine gas. Singlet oxygen is diluted with the carrier nitrogen buffer at low pressure (few tens of torr). This mixture of nitrogen and singlet oxygen is then mixed with iodine vapour (iodine being the lasing species). The mixture is then passed through the throat of supersonic nozzle and expanded into the laser cavity to achieve lasing action. The laser effluents are thereafter exhausted to an evacuation system (vacuum dumps, pumps, or both) or a pressure recovery system (ejector, cryo-sorption, cryo-condensation) LASTEC successfully developed and demonstrated its first small scale COIL system delivering 350 W power output in 2001. With a view to address the technological gap, a ground based 10 kW COIL was developed through collaborative efforts and was successfully demonstrated in 2002 through vacuum dumps. Technological gains from the earlier realised COIL systems led to successful development and demonstration of ground based direct exhaust 20 kW COIL system. Development of a COIL system is highly multidisciplinary in nature involving technological challenges. The subsystems which involve most critical technological challenges are: singlet oxygen generator, supersonic nozzle system, exhaust control system and optical resonator. LASTEC has worked extensively on jet based singlet oxygen generators which were utilised in all the three COIL systems developed indigenously. LASTEC has also worked on various versions of supersonic nozzles, viz., slit nozzle, subsonic iodine injection grid nozzle, advanced nozzle and winglet nozzle. Optical resonators with single and multiple pass geometries have also been developed and used in demonstration of high power COIL systems. Present thrust is to address the challenges in the development of some of the most critical technologies and the new concepts which form possible solutions for compact futuristic high power COIL systems. The foremost being the development of high pressure singlet oxygen generator with operating pressure in the range of 80-100 torr with high throughput operation. The other crucial technological area of research is high efficiency transonic supersonic nozzle system capable of providing higher small signal gain. LASTEC is extensively working on these critical technological areas for planned development of 30-100 kW vehicle mountable sealed exhaust COIL systems with M 2 < 2 to achieve strategic application goals.
Solid State Lasers (SSL):
At LASTEC, different types of solid state laser sources have been designed and developed in the past few years.
Eye Safe SSL:
Lasers operating around 1.5 µm have the highest permissible ocular exposures accepted by the ANSI 2.136.1-1993 standard (1 J/cm2 for single pulses). This wavelength is not only eye safe but falls in the window of atmospheric transparency and is absorbed by OH-groups and many organic substances.
Eye Safe SSL for Laser Cross-section Measurement Applications:
An intracavity OPO converted, diode pumped and solid state electro-optically Q-switched Nd: YAG laser, capable of generating 8-10 mJ, ~ 5 ns at eye safe wavelength of 1570 nm with variable divergence has been designed and developed.
High Energy High Repetition Rate Eye Safe SSL:
Two experimental prototypes of high energy eye safe lasers have been jointly developed with BI Stepanov Institute of Physics, Minsk, Belarus. One laser is based on Er:Glass laser which directly generates the eye safe wavelength 1540 nm with ~ 25 mJ energy at 20 Hz. Other laser is based on OPO shifted Nd:YAG laser and generates 50 mJ @ 20 Hz at 1570 nm.
Miniature Diode Pumped SSL:
Miniature diode pumped solid state lasers offer high power density (output power to volume ratio) and thus are desirable for most applications requiring small laser footprint. These applications include military, homeland security, instrumentation, materials processing, space and remote sensing. In addition, with proliferation of battery powered applications including laser flashlights, dazzlers, and other electro-optic modules that utilise modern high density lithium battery technology, require miniature laser sources with minimum volume and power consumption. Efficient mini lasers also minimise the volume and weight of the control unit used to provide electrical power and cooling functions to the laser head.
High Peak Power Monolithic Miniature Diode Pumped Nd: YAG Laser:
A monolithic, diode, side-pumped and passively Q-switched Nd 3+ : YAG / Cr 4+ : YAG composite laser generating ~ 3 MW peak power pulses at 20 Hz with 15 mJ energy in 5 ns duration has been designed and developed. The compact and robust laser is suitable for variety of applications including range finding, designation and Laser Induced Breakdown Spectroscopy (LIBS). Air breakdown has been demonstrated by focussing the laser output.
The prime concern for any high power laser system is the ease of generation of laser medium and its pumping accompanied with efficient thermal management ensuring a compact laser system with high efficiency and near diffraction limited beam quality. This is best achieved by means of utilising a flowing lasing medium. An approach in this direction is the use of liquid lasers. These basically employ aprotic lasing fluid impregnated with rare earth metals such as Nd3+ and Yb3+ filling the optical cavity for lasing action. The thermal issues are overcome by circulating the liquid laser media using a pump and heat exchanger assembly allowing the replenishment for desired beam quality. This is a new and upcoming technology having potential of generating hundreds of kilowatt in most compact form factor. LASTEC has also initiated work on this type of laser source. Neodymium based liquid laser would essentially consist of Neodymium (III) Phosphorous Dichloridate retained in solution with Phosphorous Oxy-Chloride (POCl3) by addition of a Lewis acid such as Zirconium Chloride (ZrCl 4). Lewis acid is desirably included in the solution to increase solubility of neodymium (III) phosphorous formed, optimise the intensity of fluorescence of the liquid solution and its operative efficiency. Also, Neodymium Trifluoraceate which is dissolved in POCl3 for generating Neodymium (III) Phosphorous Dichloridate is required to be synthesised by mixing Neodymium Oxide (Nd2O3) and Trifluoroacetatic Acid (CF3COOH) followed by evaporation of excess water. The liquid laser solution of POCl3 :Nd 3+ :ZrCl4 is highly stable over relatively long periods of time and does not degrade under flash excitation. It forms the basic part of the laser cavity.
Liquid laser system consists of a lasing chamber through which the lasing medium is circulated in a closed loop. A diode array used to excite the liquid lasing medium is located within the lasing chamber. A pump and a pressuriser along with a heat exchanger (shell in tube/ plate type), circulate the lasing fluid in a closed loop. The lasing chamber is then enclosed by the optical resonator at the two ends defining excitation volume for laser power extraction. LASTEC also plans to develop rare earth metal doped liquid laser as it is a potential high power laser source with single aperture output. The technological areas of research pertaining to this laser include the kind of usable aprotic solvent, rare earth metals impregnable into the solvent and synthesis of liquid laser solution.
OPO Based Tuneable Laser Source:
Most lasers are single wavelength devices, or have only a restricted range of multiple wavelengths due to the inherent physical properties of the gain media used. The most widely used wavelength-tuneable coherent radiation sources are dye lasers and Quantum Cascade Laser (QCL). Each dye has a rather limited tuning range of about 5 to 20 nm in the visible part of the spectrum and tenability of QCL is also limited to about 200 nm. However, lasers based on optical parametric wavelength conversion are more efficient and widely tuneable than dye lasers.
Optical Parametric Oscillator (OPO) is basically a non-linear optical device which provides tuneable radiation from fixed laser source through parametric generation. In simple form it consists of a pump laser and a nonlinear crystal in a cavity. Laser based on optical parametric processes provide wide and continuous wavelength coverage of more than 1000 nm, easy and rapid wavelength tenability, high energy output and has the inherent advantage of being all solid state. In the recent past, LASTEC has worked on various OPO systems. KTA crystals have very high non-linear coefficient and damage threshold making it possible to achieve the OPO oscillation at relatively low threshold value. Cavity is made of plane mirrors of appropriate length, with direct/ side entry of pump beam through the cavity mirror. The module can be conveniently placed in front of pump laser to get the OPO action. Output from an OPO is broader in line width. To have a narrow line width, cavity is modified to a mirror grating cavity by introducing a wavelength selective element in the cavity. A system consisting of mirror grating cavity in littrow configuration using high precision motorised rotation stage has been developed. LASTEC has designed and developed a solid state tuneable infrared source emitting in the wavelength band 2- 4 µm and 8-15 µm. A patent “Incoherent Infrared Source Tuneable from 2-18 µm” for the design has also been filed.
Here Comes The REAL Thing ~ High Power Fiber Lasers (HPFL):
Continuous Wave (CW) Fiber Laser:
Fiber lasers have become the disruptive technology in the field of lasers as they are adjudged superior to many of the established lasers for various applications. The recent advances in optical fibers and related technologies resulted in the development of High Power Fiber Lasers (HPFL) for industrial and defence applications. It has many proven advantages over other lasers. High efficiency, low thermal management constraints, small footprint, no requirement of logistic supply chain and above all good inherent beam quality make fiber lasers most suitable for platform integrated long-range DEW applications. Another major advantage of fiber lasers is that they are highly suitable for power scaling using different beam combining techniques. Development of very high average power fiber lasers (> 100 kW) is envisaged by many countries for building DEW systems for tactical military applications. As fiber laser beam is confined within the flexible fibers and requires no alignment or free space bulk optics such as lenses and mirrors, systems built using fiber lasers have significant advantage in harsh military environments. However, development of all-fiber single mode high power fiber lasers is very challenging and crucial for development of HPFL based DEW systems.
LASTEC has undertaken a TD project to develop HPFL technologies and necessary infrastructure to build 1 kW single mode fiber laser sources which can serve as basic building block for developing much higher power laser sources using different beam combining technologies.
LASTEC with the help of a collaborator has developed one unit of 1 kW single mode fiber laser with output beam quality parameter M2 value of 1.12. One more unit of 1 kW fiber laser is currently under development. Though there are several critical technologies involved, fusion splicing of all fiber components with lowest light loss is critical to make a monolithic robust structure which is alignment free and compatible for field deployment.
Pulsed Fiber Laser;
Pulsed Fiber Lasers (PFLs) are rapidly replacing the conventional Q-switched bulk solid state lasers for generating nano second pulses at multi-kHz repetition rates. Such lasers are required for various long range applications like remote sensing, imaging, LIDAR, etc. Advantages offered by fiber based solutions include simpler thermal management, higher efficiency, flexible pulse format and waveguide defined beam quality independent of power level. Fully integrated structure in fiber laser provides compact, robust, alignment free laser that is compatible with highly efficient fiber pigtailed high power pump diodes and various fiber integrated devices such as FBG, fiber couplers, etc.
LASTEC has recently initiated work on this very challenging domain of developing different pulsed fiber laser sources involving Ytterbium and Erbium doped fibers to generate 1.0 µm and 1.5 µm wavelengths respectively at high repetition rates. These sources are required for long range applications like target illumination, dazzling, etc.
To get high beam quality, single mode waveguide structures are required. A small core clad numerical aperture is used to maintain the single mode transmission but it also leads to intense power density in a small core area, which brings strong nonlinear effects such as Stimulated Raman Scattering (SRS), Stimulated Brillouin Scattering (SBS) and Amplified Spontaneous Emission (ASE), which can limit the maximum output power of fiber lasers. The use of large core or Large Mode Area (LMA) fibers helps in reducing optical intensity and prevents these parasitic effects. Recently peak powers ~1 MW have been reported in combination with multi-watt average powers. The LMA in conventional fibers, however, leads to multimode propagation leading to degradation of beam quality. Therefore novel fibers called Photonic Crystal Fibers (PCF) have been the subject of intense research in recent years.
Development of pulsed fiber laser sources would require several critical components with high damage thresholds to sustain high peak power densities. These components include LMA doped delivery fibers, high power pump laser diodes, isolators, pump combiners, couplers, filters, etc.
Significant advances in high power diode pump lasers, refinement of power scaling and energy storage techniques, fiber component fabrication such as FBG are opening up the way for the development of active fiber systems that can deliver tens of watts of single transverse and longitudinal-mode output power, millijoule pulse energies, and ultrashort pulses with peak powers in the 10-100 MW region.
The Govt’s Recent New Direction In Fiber Laser Weapon Research Initiative:
Recent activities of government prompts that govt is shifting Fiber Laser’s weapon system R&D to a new and dedicated lab which will work as an independent lab under DRDO, named Centre for High Energy Systems and Sciences (CHESS), Hyderabad and will work in tandem with the age-old LASTEC, use Laser Sources and associated technologies already developed by LASTEC and use them to device a product grade weapon system for end-user.
A DRDO document says: CHESS has a mandate for the design, development and field testing of laser based directed energy weapon system as per the requirement of user agencies. Presently Indian army has issued two PSQR’s for the development of 100kW ground mobile directed energy laser weapon systems. Towards this CHESS has undertaken a forerunner project “Development of Experimental Technology Modules for Directed Energy Laser System (TD2014/CHE-659)” for establishing critical technology modules of a directed energy laser system. The proposed system design architecture of the 10kW directed energy system (DES) is based on electrically driven single mode fiber laser. The 10kW DES is proposed to be realized through development of technologies for space beam combination of two 5kW fiber laser beams on to a distant moving target. Development of a compact light weight power conditioning system (PCS) is one of the critical technology area of this project. For 10kW DES the total electrical power requirement is of the order of 94kVA during laser operation and around 44kVA during idle running/cooling mode. The required power demands for field operation of the 10kW DES is planned to be provided by a compact light weight generator with integrated compact energy storage (Li-ION batteries) for providing a 1 min power back-up for protection of critical sub-systems Permanent magnet alternator (PMA) based diesel genset is selected for reducing the foot prints by 50% compared with conventional diesel genset In conventional diesel gensets output power is derated by 35-40% in extreme environmental conditions, (altitude above 3500m). PMA based variable rpm gensets use turbo charged diesel engine technology which have 10-12 % of de-rating at extreme environmental conditions. Current project is technology enabler to the main program, which is mobile 100kW class directed energy laser weapon system configured on two vehicles. All subsystems developed here are scale down versions of main program. Therefore power conditioning system of current project is also designed as scale down version of main program to address & mitigate the overall operational & technical complexities. Since 2x50KVA PMA based Trailer mounted power supply system is a customized development, therefore it is proposed to raise the case on open tender basis.
A recent media report from economic times said, CHESS has successfully tested a 1KW Fiber Laser system mounted on a truck at Chitradurga in Karnataka towards last August end. Quoting an official ET wrote “The laser beam hit a target located 250 meters away,” “It took 36 seconds for it to make a hole in the metal sheet.”
This is obviously a technology enabling pilot project demonstration test for the actual goal which is .
The next step is to test a higher powered laser, 2KW, mounted on the truck against a metal sheet located at a distance of 1 km.
Previously a report from media said, CHESS has developed and successfully tested a single-board High-Speed Module (HSTM) incorporating field-programmable gate array (FPGA) and digital signal processor (DSP).
The system acquires images from a high frame rate camera (>1000 fps), pixel size 512×512, using CAMLINK interface, computes the centroid of the distant target and tracks the target, and controls a two-axis Fast Steering Mirror (closed loop bandwidth ~400 Hz) for precisely maintaining the laser beam spot onto the target centroid during the period of illumination. Single frame latency and Jitter correction of 95 per cent, 90 per cent and 80 per cent has been achieved for 10 Hz, 20 Hz and 30 Hz jitter frequencies and ± 0.3 µrad jitter amplitude, respectively under lab simulation conditions.
The system would provide improved compensation of atmospheric effects on laser beam propagation in comparison to the commercial NI-based system being used presently.
While India’s Defence budget is already awfully low and DRDO gets only ~7% of the total and so LASTEC and CHESS gets less than 0.2% (DRDO have 56+ labs to feed), its already a miracle they showed the guts to go to that High Tech DEW field. Not only this, they have achieved credible success in the field.