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OPA657: OPA657 oscillation problem

Part Number: OPA657
Other Parts Discussed in Thread: OPA818

We have a laser diode-PMT system. The output of PMT module is very low signal. We use an opamp-based(OPA657) transimpedance amplifier to amplify the output of PMT. Our frequency is 15-20 MHz. But we want to reach to 100MHz (at least 50MHz).  My first observation is when we block the laser diode signal, there is oscillation signal at obvious frequencies(above 20MHz).  When the laser block diode is on, there is signal but when we block the laser diode, this signal dont interrupt. I'm confused about OPA657 causes this oscillation and if yes, how can we prevent this? I'll be appreciate if you give some advices. 

Thank you,