A Lightning to Digital Optical (Toslink) cable/adapter would be a rarity, even if it should exist.
A potential workaround would be something like the following. However, I have no way of verifying that this will work.
- Start with the Apple Lightning to USB Camera Adapter.
- Next, connect a USB DAC that has an optical digital output.
- Finally, you would use a standard Toslink optical cable to connect the DAC to your optical audio device.
The issues will, of course, be two-fold:
- The exact type of DAC you have that can support these types of connections.
- The cost of the additional equipment needed to make this work.