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Entry  Thu Oct 31 18:31:10 2019, Koji, Laser, PD QE, PD EQE vs Spot size QE_vs_spotsize.pdf
    Reply  Fri Nov 1 12:47:18 2019, Koji, Laser, PD QE, PD EQE vs Spot size QE_vs_spotsize_no_saturation.pdfQE_vs_spotsize_saturation_0_85.pdfQE_vs_spotsize_saturation_0_1.pdf
       Reply  Tue Nov 12 03:40:39 2019, Koji, Laser, PD QE, PD EQE vs Spot size QE_vs_spotsize_half_power.pdf
Message ID: 2438     Entry time: Thu Oct 31 18:31:10 2019     Reply to this: 2439
Author: Koji 
Type: Laser 
Category: PD QE 
Subject: PD EQE vs Spot size 

InAsSb PD QE Test

The relationship between the spot radius and the apparent QE (EQE) was measured.

1) The spot size was checked with DataRay Beam'R2. The beam scanner was mounted on the post with a micrometer stage in the longitudinal direction. (Attachment1 upper plot)
It was confirmed that the beam is focused down to ~22um. The incident power was about 0.9mW.

2) The InAsSb detector (Sb3513A2) was mounted on the PD holder and then mounted on the stage+post. The photocurrent was amplified by a FEMTO's transimpedance amp (V/A=1e3Ohm). The dark current and the total photocurrent were measured at each measurement point with the beam aligned to the PD every time. The estimated EQEs were plotted in the lower plot of the attachment.

Note that P2, P3, and P6 elements have the size of (500um)^2, (750um)^2, and (1000um)^2, respectively.

The absolute longitudinal position of the sensor was of course slightly different from the position of the beam scanner. So the horizontal axis of the plots was arbitrary adjuted based on the symmetry.

The remarkable feature is that the QE goes down with small spot size. This is suggesting a nonlinear loss mechanism such as recombination loss when the carrier density is high.

With the present incident power, the beam size of 100um is optimal for all the element sizes. For the larger elements, a bigger beam size seems still fine.

The next step is to estimate the clipping loss and the saturation threshold with the Gaussian beam model.

Attachment 1: QE_vs_spotsize.pdf  71 kB  Uploaded Thu Oct 31 19:32:22 2019  | Hide | Hide all
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