laser signal to electrical signal?

Yes. Use a photo-transistor. What type depends on what you want to do with it.
For fiberconnections you have special highspeed tranceiver blocks with a laser to send and a receiverblock consisting of a photo-transistor and a opamp to receive.

Your photo-transistor have to be matched with the wavelength of your laser for the best results.

what parameter of transistor must b matched with the wavelength of laser.....lets say if my laser has a wavelength of 670nm then wat kindaphototransistor do i hv to use?

Just assume any photodiode or phototransistor and forget the rest. There no relevant spectral response differences among them in near IR and visible range. The most interesting question is: Are we talking of pulse respectively high frequency modulated lasers or simple CW?

Actually I want to modulate the data on laser and for that I need a fast switching transistor. So I wanna know how many number of bits I can transfer by the switching of transistor if it has total time period of 20ns...I know the frequency I get is 50MHz...but is the bit rate 50Mbits/s