My husband is mechanical engineer & currently earns 20k/m. Are there any opportunities in this field to increase his income?.

Sure, let me do a further search on it. Thanks for the details

Ask him to explore medical equipment engineering and repairs/servicing (biomedical). He can possibly 2-3x his income.

There are many equipment dealers and distributors who need engineers for servicing and repairs. So do manufacturers and even medical colleges and hospitals.

Some companies for reference - Carl Zeiss, Leica, Huber, Thermo fisher, Dentsply Sirona, Olympus Evident, Hemant surgicals. Many more such companies in India.