Scope and Objectives

Coherent beam combining is a key pathway to achieving 100 kW power levels required for DEW applications. The basic building block for such beam combined systems is a narrow linewidth, nonlinearity managed kW-class laser with excellent beam quality (M-squared < 1.2). In this project, we will develop such basic building blocks (IISc) as well as demonstrate coherent beam combining of such narrow linewidth lasers (IITM) with the target of achieving >80% beam combination efficiency and M-squared < 1.5.

Deliverables

A packaged, high power fiber laser module amenable to power combining with output power of > 500W, single polarization, linewidth< 10 GHz (< 0.04 nm) and beam quality M-squared < 1.2. Demonstrate coherent beam combining of above narrow linewidth laser units with 80% beam combination efficiency and M-squared < 1.5.

Videos

Scientific Output

The work done during the past one year are outlined below. Under the first task of modelling and design, effect of thermal mode instabilities (TMI) as well as stimulated Brillouin scattering (SBS) and their compensation mechanisms in high power narrow linewidth fiber lasers have been studied. Though the phase control method used in the proposed experimental setup is LOCSET, stochastic parallel gradient descent (SPGD) algorithm is used to control the phases of the individual laser beams for coherent beam combining. The SPGD method is more robust against phase fluctuations and less complex. A diffractive optical element based coherent beam combing methodology is considered to scale up the fiber laser power far above the limitation of single fiber laser by manipulating the wavefronts of the multiple beams. The line-broadening architecture for the seed source has also been finalized. The techniques for the measurement of the broadened lineshapes as well as the input and drive power configurations for complete suppression of the carrier have been identified.

Results and outcome till date

CBC using Diffractive Beam Combining Element An experimental setup for coherent combining of 2x100W High-power laser beams using a diffractive beam combining element (DBCE) has been conceptualized and implemented as shown in Fig. 1. The DBCE is a bidirectional element, which works as a splitter/combiner depending on the direction of propagation. The key components of the optical setup are the two high-power collimated laser beams (CL1 and CL2), Mirrors (M1-M6), Apertures (A1-A7), DBCE, Photo detector, CCD and the Laser Power meter. All the components used in the experimental setup will be placed on the vibration isolation tables so as to avoid the ambient mechanical vibrations. Collimated High-power laser beams are allowed to fall on the DBCE with a precise angle of 4.13deg. using the tilting mirrors (M5 & M6). The DBCE will combines the beam coherently. The beam power is measured by the laser power meter along to measure the efficiency. The time trace of the optical power at the CBC output using the DBCE, without and with phase controlling with the SPGD, is shown in Fig. 2. The combining efficiency is analysed and found to be close to 75% with 0.11 rms. The stabilized combined-beam using real-time control is shown in Fig. 2 whereas PSF of combined beams is recorded and given in inset of Fig. 2; the experimentally measured M2 parameter is found to be 1.39. Ongoing efforts are focused on the combining of 4x500 W laser beams using the DBCE with improved efficiency.

Societal benefit and impact anticipated

The coherent beam combined high power laser is expected to contribute significantly to our country's defence. In addition, it may be used as a source for long range LIDAR through which a variety of applications ranging from pollution monitoring to climate modelling is enabled.

Next steps

1. The next steps for the narrow-linewidth laser source are as follows a) Process optimization and build of the 500W class power amplifier b) Coupling the linewidth tunable 20W seed source with the power amplifier to demonstrate the complete narrow-linewidth PM laser system. 2. In the next stage of the phase controlling part, Implementing SPGD in the Mach-Zehnder setup with two low power YDFAs on both arms and combine the optical power in the arms using couplers and in free space (using collimators). Design of DBCE element has been finalized. DBCE is evaluated for its overall performance in the final experiment with the help of Virtual Lab and Solid works model. The diffractive element is in process to purchase. With the help of DBCE, the final experiment for combining the laser beam will be demonstrated.

Publications and reports

1. Panbiharwala, Y., Harish, A.V., Venkitesh, D., Nilsson, J., Srinivasan, B., Investigation of temporal dynamics due to stimulated Brillouin scattering using statistical correlation in a narrow-linewidth CW high power fiber amplifier, Optics Exp. 26, 33409-417 (2018).
2. Panbiharwala, Y., Ghosh, A., Nilsson, J., Venkitesh, D., Srinivasan, B., Experimental investigation of the onset of modulation instability as a precursor for the stimulated Brillouin scattering in Yb-doped fiber amplifiers. In SPIE Proc. Fiber Lasers XV: Technology and Systems, Vol. 10512, p. 105122X (2018).
3. Dixit, A., Venkitesh D., Srinivasan, B., Design and Analysis of Beam Propagation Model for Coherent Combining of High Power Laser Beams. In Proc. of International Symposium on Optics, IIT Kanpur, Kanpur, India, pp. 308-309 (2018).
4. Linslal, C. L., Sooraj, M. S., Padmanabhan, A., Venkitesh, D., Srinivasan B., Implementation of Stochastic Parallel Gradient Descent Algorithm for Coherent Beam Combining. SPIE 10811, p. 1081115 (2018)
5. Srinivasan B., Linslal, C. L., Dixit, A., Sooraj, M. S., Challenges in Coherent Beam Combining of High Power Narrow Linewidth Fiber Lasers. In Proc. of International Conference on Fiber Optics and Photonics, IIT Delhi, New Delhi, India, pp. TF1-11 (2018).
6. Velpula B., Prakash R., Choudhury V., Aparanji S., Vikram B. S., Supradeepa V. R., Experimental analysis of stimulated Brillouin enhancement in high power, line-broadened, narrow-linewidth fiber amplifiers due to spectral overlap between the Brillouin gain spectrum and the signal back-scatter from the fiber termination. (Accepted in SPIE Photonics West 2019).
7. Vikram B. S., Choudhury V., Prakash R., Aparanji S., Supradeepa V. R., Continuously linewidth tunable, polarisation maintaining, narrow linewidth fiber laser. (Accepted in SPIE Photonics West 2019).
8. Choudhury V., Aparanji S., Prakash R., Velpula B., Supradeepa V. R., Observation of visible light flashes in high power, near infrared, narrow-linewidth fiber lasers and its potential use as a visual monitor for stimulated Brillouin scattering. (Accepted in SPIE Photonics West 2019).
9. Panda B., Velpula B., Raj P., Mallick M., Supradeepa V. R., A simple technique for direct, high power laser beam profile measurement using thermal imagers. (Accepted in SPIE Photonics West 2019).
10. Linslal C.L., Sooraj M. S., Panbiharwala Y, Padmanabhan A, Dixit A, Deepa Venkitesh, Balaji Srinivasan, Investigation of Line Broadening Scheme Dependence on Coherent Beam Combination Efficiency. (Accepted in OSA Laser Congress 2019).

Patents

None

Scholars and Project Staff

Dr. Linslal, Postdoc
Dr Awakash Dixit, Postdoc
A. Padmanabhan, Research Scholar
M.S. Sooraj, Research Scholar
Vidya Mohan, Project Manager
Sivasubramanian, Project Associate
Prakash, Project Technician
Jasmine, Project Technician
Vaithya Devi, Project Technician

Challenges faced

Primary difficulty in the project is securing timely release of funds. Presently, for the second year we received funds from MHRD but yet to receive the funds from DRDO.

Other Information

Dr Supradeepa has been successful in developing a 500 W fiber amplifier module. We are trying to order all the components for 3 more such modules. However, our plans have been hampered since we are yet to receive the DRDO component of the second year funding. Kindly recommend the release of the funding at the earliest.