Dear all,

I perform a phase modulation technique of an optical signal. The amplitude of my optical signal is modulated at fm = 1GHz and reads sm*cos(2pi*fm*t). The signal passes through a sample and I am interested in the resulting phases (phi) acquired such as sm*cos(2*pi*fm*t+phi).

The phase (phi(t)) varies quickly in roughly 10 ns. This variation is simultaneous with a variation of the amplitude (sm(t)).
What I am interested in is the phase phi(t). The problem is that both the phase and the amplitude vary quickly in time, what broadens my signal in frequency.

I perform the detection with a detector with a cut-off frequency of 2.5 GHz. Since my signal is kind of broad (roughly 10 - 100 MHz around 1 GHz), the phase that I measure in the time domain (using a RF scope to measure the detected signal in time) is the superposition of the phase that I want and an extra phase from the detector (convolution of the broad spectrum and the phase response of my detector). Indeed the phase change (phi(t)) that I want to see if rather small (few degrees), while I picture that by working so close to the cut-off of my detector the residual shift would be about tens of degrees.

Since I cannot change either my carrier frequency or my detector, I am looking for an electronic method to get rid of the residual phase coming from my detector. I was told that an IQ demodulation might do the trick. By IQ demodulation I mean multiplying the signal by cos(2*pi*fm*t) and sin(2*pi*fm*t) and combining the two to extract independently the amplitude sm(t) and phi(t).

Can someone confirm that this technique will get rid of the influence of my detector? Otherwise is there a way to retrieve the contribution from the detector?

Best

 

Dear all,

I perform a phase modulation technique of an optical signal. The amplitude of my optical signal is modulated at fm = 1GHz and reads sm*cos(2pi*fm*t). The signal passes through a sample and I am interested in the resulting phases (phi) acquired such as sm*cos(2*pi*fm*t+phi).

The phase (phi(t)) varies quickly in roughly 10 ns. This variation is simultaneous with a variation of the amplitude (sm(t)).
What I am interested in is the phase phi(t). The problem is that both the phase and the amplitude vary quickly in time, what broadens my signal in frequency.

I perform the detection with a detector with a cut-off frequency of 2.5 GHz. Since my signal is kind of broad (roughly 10 - 100 MHz around 1 GHz), the phase that I measure in the time domain (using a RF scope to measure the detected signal in time) is the superposition of the phase that I want and an extra phase from the detector (convolution of the broad spectrum and the phase response of my detector). Indeed the phase change (phi(t)) that I want to see if rather small (few degrees), while I picture that by working so close to the cut-off of my detector the residual shift would be about tens of degrees.

Since I cannot change either my carrier frequency or my detector, I am looking for an electronic method to get rid of the residual phase coming from my detector. I was told that an IQ demodulation might do the trick. By IQ demodulation I mean multiplying the signal by cos(2*pi*fm*t) and sin(2*pi*fm*t) and combining the two to extract independently the amplitude sm(t) and phi(t).

Can someone confirm that this technique will get rid of the influence of my detector? Otherwise is there a way to retrieve the contribution from the detector?

Best

Can you make a graph of amplitude and phase vs frequency?

 

I'm not sure I get your meaning. Are you trying to PM modulate an optical signal with a 1 GHz local oscillator?

 

The IQ demodulator is a coherent demodulator, which means that the frequency and phase must be known from the local oscillator, otherwise you will have errors due te frequency and phase variations..

Not sure, but you may need to perform a phase estimation ?

 

The AD8302 phase detector chip might be suitable for your application.
The pre-sample 1 Ghz frequency is applied as the A-input, and the post-sample, phase shifted, 1 Ghz frequency is the B-input. A DC output voltage, proportional to the phase difference, is the result.

Does this description correspond to your objective?

http://www.analog.com/media/en/technical-documentation/data-sheets/AD8302.pdf