1.6V SSR power supply regulation
Normal practice is to feed the optical input (the LED) via a series dropper resistor. You know it will develop 1.6 volts at 10 mA so, if feeding from a 5 volt signal you want 3.4 volts to drop across the series resistor hence, the resistor should have a resistance of at least: -
$$\dfrac{\text{3.4 volts}}{\text{10 mA}} = \text{340 ohms}$$
To be on the safe side, choose 390 ohms. To be even safer, link the data sheet for the SSR and I'll double-check things.