C4758 View Datasheet(PDF) - Unspecified

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C4758

PSD (POSITION SENSITIVE DETECTOR)

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Characteristic and use

4. Position detection error

Position detection capability is the most important charac-

teristic of a PSD. The position of a spot light incident on the

PSD surface can be measured by making calculations

based on the photocurrent extracted from each electrode.

The position obtained here with the PSD is the center-of-

gravity of the spot light, and is independent of the spot light

size, shape and intensity.

However, the calculated position usually varies slightly in

each PSD from the actual position of the incident light. This

difference is referred to as the “position detection error” and

is explained below.

If a light beam strikes the electrical center of a PSD, the

signal currents extracted from the output electrodes are

equal. When this electrical center is viewed as the origin,

the position detection error is defined as the difference be-

tween the position at which the light is actually incident on

the PSD and the position calculated from the PSD outputs.

Figure 4-1 Cross section of PSD

SPOT

X1

LIGHT

RESISTANCE LENGTH LX

Xi

ELECTRICAL

CENTER B

Xm

X2

P-TYPE

RESISTIVE LAYER

I LAYER

N LAYER

COMMON ELECTRODE

ACTUAL POSITION Xi

CALCULATED POSITION Xm

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In Figure 4-1 above, if the actual position of incident light

is Xi and the position calculated by the photocurrents

(IX1 and IX2) from electrodes X1 and X2 is Xm, then the

difference in distance between Xi and Xm is defined as

the position detection error as calculated below.

Position detection error E = Xi - Xm [µm] ........ (4-1)

Xi : Actual position of incident light (µm)

Xm: Calculated position of incident light (µm)

Xm = IX2 - IX1 . LX ........ (4-2)

IX1 + IX2 2

The position detection error is measured under the follow-

ing conditions.

· Light source : λ=890 nm

· Spot light size : φ200 µm

· Total photocurrent: 10 µA

· Reverse voltage : Specified value (listed in data sheets)

Figure 4-2 shows the photocurrent output example from

electrodes of a one-dimensional PSD with a resistance

length of 3 mm (S4583-04, etc.), measured when a light

beam is scanned over the active surface. The position de-

tection error estimated from the obtained data is also

shown in the lower graph.

Figure 4-2 Photocurrent output example of one-

dimensional PSD (S4583-04, etc.)

IX1

IX2

1.0

0.5

0

-1.5

0

+1.5

POSITION ON PSD (mm)

Position detection error example of one-

dimensional PSD (S4583-04, etc)

+50

0

-50

-1.5 -1.0 -0.5 0 +0.5 +1.0 +1.5

POSITION ON PSD (mm)

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Specific area for position detection error

The light beam position can be detected over the entire ac-

tive area of PSD. However, if part of the light beam strikes

outside the active area, a positional shift in the center-of-

gravity occurs between the entire light beam and the light

spot falling within the active area, making the position

measurement unreliable. It is therefore necessary to select

a PSD whose active area matches the incident spot light.

Figure 4-3 Center-of-gravity of incident spot light

ACTIVE

AREA

SPOT

LIGHT

OUTPUT

ELECTRODE X1

CENTER-OF-GRAVITY

OF SPOT LIGHT FALLING

WITHIN ACTIVE AREA

CENTER-OF-GRAVITY

OF ENTIRE SPOT LIGHT

OUTPUT

ELECTRODE X2

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7

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