LCOS-SLM characteristics

Diffraction efficiency

Definition

Diffraction efficiency is defined as the expression that represents the ratio of the 1st-order diffraction light intensity (I1st) to the 0th-order light intensity (Iave) when a phase pattern simulating a brazed diffraction grating is displayed on the LCOS-SLM. In the case of a two-level diffraction grating, however, the average value of the ±1st-order light intensity is viewed as the diffraction light intensity. The 0th-order light intensity is the average value obtained by changing the input gray level from 0 to 255.

Diffraction efficiency = I1st / Iave [%]

Measurement method

The 1st-order diffraction light intensity is measured using a phase modulation optical system as shown in the figure where the polarization direction of the incident light is aligned with the orientation of the liquid crystal molecules. The incident angle θ is set within 10 degrees and the aperture size is set to 10 mm in diameter. Four types of diffraction gratings with 2 to 16 levels are used for the display.

Measurement setup for diffraction efficiency

Phase distribution of diffraction grating

Measurement results

We measured diffraction efficiency using light sources with wavelengths listed in the table below. The results are plotted in the graph below along with the theoretical values. The spatial frequencies of each diffraction grating for display are as follows: 2 levels: 40 lp/mm, 4 levels: 20 lp/mm, 8 levels: 10 lp/mm, and 16 levels: 5 lp/mm.

Wavelength of light sources for diffraction efficiency measurement

Type No.

Wide wavelength band type

Specific wavelength type 

-01

-07

-08

-02

-03B

-05

-12

-13

-15

-16

-19

Wavelength of light source (nm)

633

1064

1064

785

1064

407

940

532

1550

532

1950

Type No

High-power laser type

Laser metal processing type

-02L/-02R

-03BL/-03BR

-12L/-12R

-15L/-15R

-16L/-16R

-19L/-19R

-03CL/-03CR

Wavelength of light source (nm)

785

1064

940

1550

532

1950

1064

Measurement results for diffraction efficiency (typical examples) and theoretical values

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