Output Voltage of Arduino decreases

I am generating a PWM signal for switching purpose , but for driving the Mosfet(IRF 540) I am using a TLP250 ic .
The output of Arduino board is given to TLP250 and then the ouput of TLP250 is given to the switch(Mosfet) .
But the output voltage of Arduino PWM pins decreases to 2.5 volt after connecting to TLP250 , if I am measuring without connecting to TLP250 then the voltage is 5 volt.
What is the reason behind it?
Is there any problem with hardware?

The input to the TLP250 is an LED. If you want to connect it to a 5V output you need a current limiting resistor like you would for any other LED. The typical Forward Voltage Drop of your LED input is 1.6V and the LED current rating is 20 mA. A resistor that passes 20 mA at 3.4V (5-1.6) would be about 170 Ohms (3.4/.020). 200 Ohms should work fine.

Thank you sir..

Bear in mind you've already overloaded both the Arduino pin and the opto-coupler LED,
there is a chance they are no longer working properly...

Bear in mind that 20mA is the maximum current you should allow from the Arduino to flow into the TLP250. You should not run it at this current. The minimum current that is guaranteed to give reliable operation is 5mA, and the recommended current is 7 to 10mA. So I suggest you use a 330 or 470 ohm series resistor.