USB-Optical Converter....

[macuniverse]

iFi makes one ($265 from Music Direct):

 

micro – iLink

 

I haven't used this product but I have been been very happy with their DACs as well as the support they have provided.

I do like iFi products as well but, if you're planning to use this converter playing PCM as well as DTS/AC3 (i.e., feed your HT unit) ask before buying. I tried my iDSD micro DAC as a USB to Coax bridge and it only passes through 2ch PCM audio.