Pioneer PD-6050 Instrukcja serwisowa - Strona 8

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2. OPTICAL ! W H IN THE PICK-UP

2 4 OPTICAL PATH AND OPTICAL PARTS

2-2 FEATURE OF EACH PART

DISC

Half mirror

Objective lens

Reflecting mirror

Grating (diffraction grating)

Laser diode

Photo diode (with pre-amplifier)

Fig. 2-1 shows the configuration of

this pick-up's optical part

The wavelength of the light emitted from the laser diode is

between 780 and 790 nm. The light is barely visible. This

light source is spread into an ellipse from an ultra-small

emission point. The light expands at a set angle.

The emitted light goes through a grating and is divided into

three beams of 0 step and ±1 step.

The other beams of ±2, 3, and n steps are also present, but

are lost and not used. When the light reaches the half

mirror, 50% is reflected. The remaining light permeates the

half mirror and is lost.

The light then goes to the reflecting mirror where all the

light is reflected to the objective lens (finite type).

Since this pick-up's objective lens uses a finite system

(finite because the LD's convergence distance is finite), a

collimator lens is unnecessary. The old models objective

lenses are called infinite type. The light that is converged on

an ultra-small diameter spot by these objective lenses is

reflected by the disc and returns to the objective lens. Then

it goes through the half mirror where 50% of it returns to

the laser diode. The remaining 50% of light goes through

and reaches the photo diode.

This has been a general outline of the optical path. The

features of each part are explained in the following section.

+1 order

0 order

( A )

-1 order

Fig. 2-2

( B )

(1) Laser diode (LD)

The size of previously-used LDs was 90. However, a newly-

developed LD with a size of 5.60 has been introduced. This

has resulted in a compact and lightweight optical path.

(2) Objective lens

The collimator lens has been replaced by the finite objec-

tive lens which has a finite convergence distance for the

LD's optical path. This has resulted in lower costs while

preserving high performance.

The finite objective lens, like the conventional infinite lens,

is a high-performance lens designed to attain sufficient op-

tical performance even when the optical parts are not

parallel within the optical path.

(3) Half mirror

The light that returns to the objective lens goes through the

half mirror. Since the half mirror is a glass plate, it is known

that astigmatism is created for the light which enters at an

angle. The old model similarly used a glass plate and had a

device in its optical part to cancel this astigmatism.

Whereas, this new pick-up uses the astigmatism advantage-

ously for the focus servo.

Consequently, the multi-lens used in previous models has

not been incorporated in this new pick-up. This has resulted

in lower costs while preserving high performance. At the

same time, the points of parts have been reduced, improv-

ing dispersion and reliablity.

(4) Axle-sliding actuator

The position accuracy of the objective lens is an important

factor for the optical pick-up. The pick-up has a sliding axle

for the actuator which drives the objective lens. Accurate

and stable positioning of the objective lens is thus attained,

resulting in stable trackability. Also, a smooth frequency

response with low resonance is also realized as with the con-

ventional spring-supported type.