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Message ID: 2018     Entry time: Sun Feb 14 01:04:07 2016
Author: KojiN 
Type: Misc 
Category: PD QE 
Subject: Measurement of PD reflectivities and PD efficiencies for P-pol. and S-pol. 

According to the polariztion measurement, we found that the laser is not linear polarization.

Thus the QWP #2 was placed after collimator #2 as shown in Fig. 1 and the linear polarization laser was obtained.

Then the PD, C30665GH, with the glass window was set on the PD mount and placed at the center of the rotational stage as shown in Fig. 1.

The PD position is 5 cm after the beam waist and, at this position, the beam sizes in x-direction and y-direction are 215 um and 210 um, respectively.

In this setup, there are two reflected lights which may be come from the glass window and the PD.

Changing the laser incident angle to the PD, I measured the PD reflectivities and the PD efficiencies for P-pol. and S-pol.

The results are shown in Fig. 2 and Fig. 3.

The reason why the measurement end before the tasty part like ~40deg is that the beam is clipped.

Possibly, the light may already be clipped at ~25deg.

The alignment, i.e. the centering of the light to the PD, was optimized before the measurement as possible as we could.

The measurement was done only by rotating the rotational stage, i.e. I didn't use the steering mirror.

(Perhaps, I should use the steering mirror for optimizing the alignment at every angle.)

The incidnet power was 11.4 +/- 0.1 mW.

I meausred the powers of the one light, the other light (with the Iris #3), and the two lights (without the Iris #3) at all incident angles.

The errors in Fig. 2 is determined by the error of the measured incident and reflected light power. The position dependency on the power meter is not took into consideration.

When the PD responsivity for 1064 nm and the PD calibration constant (W/V) are 0.78 A/W and 5.8*10^(-3) W/V (see elog:2017), respectively, is assumed, the PD EQE is about 0.9 between -30 degree and 40 degree in the incident angle, according to

 Q = \frac{I \ h\ c}{\phi\ n\ e\ \lambda},

where I is the photocurrent, h is Planck's constant, c is the speed of light, phi is the incident power, n is the index of refraction of air, e is the elementary electronic charge, and lambda is the wavelength of the laser.

For calibrating the output voltage to EQE, the electric current must be known but, so far, I don't know the resistance in the photo detector circuit because the circuit is not made by myself. Thus I must investigate the circuit.

This is why I used "0.78 A/W" and "5.8*10^(-3) W/V" to estimate the current for a moment. The calibration constant "5.8*10^(-3) W/V" is determined without taking the errors, such as the position dependency on the power meter, the fluctuation of the laser light and so on, into consideration. (Of course, this is a problem.)

The errors in Fig. 3 is determined by the error of the reading error of the oscilloscope and the incident power. The position dependency on the power meter is not took into consideration.

Fig. 1 Current setup.
Fig. 2 Reflectivities for P-pol. and S-pol. between -25 degree and 30 degree in the incident angle. "Beam 1+2" means the measured power of the both reflected light and "Beam1+Beam2" means the sum of  "Beam1" and "Beam2".
Fig. 3 PD output voltage for P-pol. and S-pol. in the wide incident angle.


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