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Entry  Thu May 30 15:00:28 2013, Koji, Optics, General, QPD alignment 
    Reply  Fri May 31 05:46:54 2013, Koji, Optics, General, QPD alignment QPD1.JPGQPD2.JPGQPD_calib.nb.zip
Message ID: 133     Entry time: Fri May 31 05:46:54 2013     In reply to: 132
Author: Koji 
Type: Optics 
Category: General 
Subject: QPD alignment 

Peter F suggested to check the bottom surface of the PD housings if there is any protrusion/interference/whatever.
And that was true! It was found that the front side of QPD1 (Left) was lifted by a machining burr.
It seems that this burr consistently exists as the other one also have it (see QPD2 picture (right)) although it is not too terrible compared to the one in QPD1.

QPD1.JPG QPD2.JPG

Once these burrs were removed, the spots were found on the right position of each diode.
From the measurement of the power on each segment, the positions of the spots were estimated. (listed in the table)
They indicate that the spots are within 0.1mm from the center. This is good enough.

The quantum efficiency was measured from the incident power and the sum output. It seems that there are
some difference between the diodes. The numbers are consistent with the measurement the other day.

QPD#              QPD1       QPD2
Diode#            #43        #38

-------------------------------------
Power Incident     84.7 uW   86.2 uW
Sum Out            56   mV   61   mV
Vertical Out       -6.8 mV   10   mV
Horizontal Out      4.2 mV    8.8 mV
SEG1              -17   mV  -15   mV
SEG2              -14.5 mV  -11   mV
SEG3              -11   mV  -15   mV
SEG4              -13   mV  -20   mV
-------------------------------------
Spot position X   +25   um  +46   um  (positive = more power on SEG1 and SEG4)
Spot position Y   -42   um  +46   um  (positive = more power on SEG3 and SEG4)
-------------------------------------

Responsivity[A/W] 0.66      0.71
Q.E.              0.77      0.82
-------------------------------------

Arrangement of the segments
View from the beam
/ 2 | 1 X
|---+---|
\ 3 | 4 /

---------------

I(w,x,y) = Exp[-2 (x^2 + y^2)/w^2]/(Pi w^2/2)

(SEG_A+SEG_B-SEG_C-SEG_D)/(SEG_A+SEG_B+SEG_C+SEG_D) = Erf[sqrt(2) d/w]

d: distance of the spot from the center
w: beam width

Attachment 3: QPD_calib.nb.zip  21 kB  Uploaded Wed Oct 16 18:16:50 2013
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