Low-voltage rectification
A bridge rectifier may not work properly because:-


Full wave rectifying a square wave results in an almost constant DC output, not the doubled frequency that you want.


A square wave can be shaped into a more useful waveform by low pass filtering. However if your audio source is bandwidth limited then your 'square' wave may already be a sine wave.


Even with a perfect lossless rectifier you may struggle to get sufficient amplitude. A 2Vpp signal peaks at 1V, but most infrared LEDs require at least 1.1V to produce useful output. You need some way to increase the amplitude, or add a bias voltage to the signal. 


If an external power supply is not available and you must power the LED directly from the audio signal then amplifying the voltage will be difficult. However developing a bias voltage is relatively easy. 

The following circuit has positive and negative half-wave rectifiers (D1 and D2) with low value filter capacitors creating a small DC bias voltage with high ripple. High ripple is usually bad, but in this case we want the ripple because it is the signal! 

On positive half cycles C1 charges up to about 0.6V, then discharges (through the LED) to about 0.4V during negative half-cycles. C2 performs the same function but on opposite half-cycles. This bias voltage is enough to make up for rectifier voltage drop and provide a small boost to get into the LED's operating voltage range. R1 and R2 reduce loading on the input, and limit diode current if a larger signal voltage is applied.           







simulate this circuit &ndash; Schematic created using CircuitLab