# PIN Photodetector

1. Sep 4, 2009

### roujhan

I'm trying to design a fiber optic sensor to sense vibration via intensity modulation.
I'm new in this field and i don't know whether I can use a PIN photodetector for low frequency or not... what's the bandwidth of a PIN Photodetector? Does it mean its responsivity is limited in low frequency? Could you please tell me what does the "bandwidth" mean exactly in PIN Photodetector?

2. Sep 4, 2009

### Bob S

Are you talking about the optical (spectral) bandwidth (light frequency or wavelength(~ 400 to ~1000 nm)) or the modulating frequency bandwidth (dc-~250 MHz)?

3. Sep 4, 2009

### roujhan

Dear Bob;

I mean the modulating frequency bandwidth. e.g. the second page of the attached file mention the Low and high Cut-off Frequency.

Or: Please see the "bandwidth" in :
http://qphotonics.com/catalog/Fiber-coupled-InGaAs-PIN-photodiode-p-180.html [Broken]
this point to minimum and maximum bandwidth. does it mean in lower frequency, the responsivity is reduced? or... what?

I've read somewhere that in low frequency the PIN Photodetector acts as usual photodetector. Is it true? So why in the papers it have been mentioned that high speed PIN photodetectors are used for detecting signals only about 300KHz or so...

#### Attached Files:

• ###### TPT48xx.pdf
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4. Sep 4, 2009

### Staff: Mentor

I don't understand the high 3dB point of 1MHz for a 1.25Gbps receiver, but the low 3db point is probably there to help limit noise at the output. Since the receiver is optimized for modulation frequencies up around 1GHz, there is no need to pass low-frequency content, which only adds to noise with no signal content.

And yes, a PIN photodiode has response down to DC. It would be other circuitry in the receiver that is limiting the low-frequency response of this module.

5. Sep 4, 2009

### Bob S

hi roujhan-
These units are built primarily for telecommunications applications, and have built-in transimpedance amplifiers that are ac coupled to the PIN-diode. For your application, because of the ac coupling, you would have to use a ~1 MHz- 100 MHz carrier illumination which the vibration would phase modulate. For a low frequency intensity-modulated signal, you probably should use a dc-coupled photodetector. What is the vibration frequency, and how does the vibration intensity-modulate the signal?
Bob S

6. Sep 4, 2009

### Bob S

Hi roujhan-
Here in the thumbnail is a list of PIN diodes available from www.digikey.com at about $1.00 to$10.00 each.
Bob S

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7. Sep 5, 2009

### roujhan

Thanks a million Bob,
about the method vibration modulates the light intensity:
I want to use FBG (Fiber Bragg Grating) that its Bragg wavelength suffer shift when strain is applied to. since the vibration in fact is a dynamic strain, so with vibration the Bragg wavelength will change in time. with a filter or another FBG as a demodulator the wavelength shifts convert to the intensity modulation.