While writing hdl code you cannot specify time values and even if it simulates it is not synthesizable.
You can generate pulse fo 30 micro sec as
constant pulse_period : time := 30000 ns;
pulse <= 0;
pulse <= not pulse after 40000 ns;
pulse <= not pulse after time;
or you can use wait statements also
as
pulse <= 0;
wait for -----(specify time here);
pulse <= not pulse;
wait for pulse_period;
pulse <= not pulse;