Capacitive DOE integrity monitor

A method for producing an optical module, the method comprising:

• providing first and second transparent substrates;
• forming first and second DOEs on respective faces of the first and second transparent substrates;
• forming first and second transparent conducting electrodes over the first and second transparent substrates, respectively, such that the DOEs and electrodes are in mutual proximity;
• bonding together the first and second transparent substrates to form a bonded substrate pair in which the first and second DOEs are in mutual alignment, with the first transparent substrate in proximity to and parallel to the second transparent substrate;
• and
• coupling circuitry to measure a capacitance between the first and second capacitance electrodes.