HMC799 photodetector oscillation at 1.6GHz?

With the attached schematic and PCB I get an expected photodiode-response from around 0 to 400 MHz. However with the detector dark (or illuminated) the HMC799 seems to self-oscillate at around 1.6 GHz with a measured power of -27 dBm (attenuated perhaps 3-6 dB by the BUF602 I am using).

Any suggestions that I could try with this PCB, or design changes that I could make to the next version of this PCB?

  Trace A is the SA floor. Trace B is the detector powered but not illuminated. Trace C is the optical response from a modulated VCSEL light source, showing 400 MHz optical bandwidth. The photodiode is either FDS015 or FDS025 from Throlabs. Changing the photodiode changes the shape of the optical response slightly but not the 1.6 GHz oscillation.

 This is the schematic I am using. Are there better ways of applying a DC reverse bias (to reduce the photodiode capacitance) while AC-coupling the signal to the HMC799 input?

  2-sided PCB top side. There are 9 vias on the EP of the HMC799. 

 PCB bottom side.

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