We present a search for optical bursts from the repeating fast radio burst FRB 121102 using simultaneous observations with the high-speed optical camera ULTRASPEC on the 2.4-m Thai National Telescope and radio observations with the 100-m Effelsberg Radio Telescope .
A total of 13 radio bursts were detected , but we found no evidence for corresponding optical bursts in our 70.7-ms frames .
The 5- \sigma upper limit to the optical flux density during our observations is 0.33 mJy at 767nm .
This gives an upper limit for the optical burst fluence of 0.046 Jy ms , which constrains the broadband spectral index of the burst emission to \alpha \leq - 0.2 .
Two of the radio pulses are separated by just 34 ms , which may represent an upper limit on a possible underlying periodicity ( a rotation period typical of pulsars ) , or these pulses may have come from a single emission window that is a small fraction of a possible period .