JPH03214102A - Optical coupler - Google Patents

Abstract

PURPOSE:To decrease the generation of noise by the return light to a laser by confining the reflectivity of the light incident surface of a prism to <=25%. CONSTITUTION:The light generated from a semiconductor laser diode LD is reflected by the light incident surface 2a of the prism 2 and an optical disk 5 having bit information is irradiated with this light via a lens system 4. The reflected light modulated according to this information is introduced from the light incident surface 2a of the prism 2 into the prism 2 and is introduced to plural sets of photodiodes PDs on an optical IC substrate 1. The reflectivity of the light incident surface 2a of the prism 2 as an optical coupler is confined to <=25%. The noise is decreased in this way, i.e. the S/N is improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is an optical coupler for a laser beam for an optical integrated circuit, for example, and is used in an optical pickup for reading out recordings on an optical disk, a so-called laser disk, a compact disk, etc. using a laser beam, for example. Related to optical couplers.

[Summary of the invention]

In particular, in an optical coupler for laser light using a prism, the present invention significantly reduces noise generation due to return light to the laser by setting the reflectance of the light incident surface of the prism to 25% or less.

[Prior Art] As an optical pickup for reading bit information recorded on an optical disk, such as a so-called compact disk or a laser disk, by irradiating a laser beam, an optical integrated circuit ( optical IC)
This optical IC board (1) is placed on the optical IC board (+) containing
A prism (2) constituting an optical coupler is mounted while maintaining a required positional relationship with the photodiode PD formed in the photodiode PD. Furthermore, a semiconductor knob (3) is mounted on this optical IC board (1). This semiconductor chip (3
) is attached with a semiconductor laser LD, and is further formed with a power monitoring photodiode PD having, for example, a PIN structure, which detects light directed backward from the laser LD and controls the power of the laser LD.

The light generated from the semiconductor laser diode LD is reflected by the light incident surface (2a) of the prism (1) and passed through the lens system (4) to the optical disc (5) containing bit information.
The light modulated according to this information is introduced into the prism (2) from the light incidence surface (2a) of the prism (2), and the plurality of sets of photodiodes P on the optical IC board (1) are
Introduce or couple to D, thereby
The recording information modulated laser beam is detected to read signals, and servo signals such as tracking servo signals and focus servo signals are extracted.

In this case, the prism (2) as an optical coupler has a half-mirror function on its entrance surface (2a), that is, an inclined surface having a required angle, and directs the laser light from the semiconductor laser LD to the optical disk (5). The optical disc (5)
It is necessary to introduce the light reflected from the prism (2) into the prism (2) in order to efficiently direct it to the photodiode PD.

Therefore, taking both functions into consideration and not considering light absorption,
The reflectance R3 of this entrance surface (2a) is selected to be approximately 50%. That is, the reflectance R3 at the entrance surface (2a) of the prism (2) and the amount of light S returning to the semiconductor laser LD.
, and the light reflected from the optical disk (5) to the photodiode PD, that is, the signal light amount So, is as follows. Therefore, the ratio S (%) of the reflectance R3 and each light amount is
) are as shown in curves (31) and (32) in FIG. Conventionally, R+ is generally set to 50% in principle so that the signal light amount S0 is maximized. Therefore, in this case, the return light Sf theoretically reaches as much as 25%. Therefore, in a semiconductor laser LD using this type of optical coupler, a gain guide type laser is generally used in which the influence of the returned light on noise is small, that is, a laser with low coherence due to multimode oscillation is used. However, in recent years, laser L pickups have become smaller and lighter, for example in the optical pickups mentioned above.
The external cavity length, that is, the optical path length L ext of the laser beam emitted from D is shortened, and L ! When Xt becomes about 20 mm, noise due to the return destination to the laser LD becomes impossible to ignore.

That is, FIG. 2 shows the optical path length, that is, the external cavity length of the laser, 8. As is clear from the relationship between and S/N, when L IIXL becomes 20 mm, the noise becomes significantly large and the S/N decreases. In this case, the interval between the noise valleys (worst value) is the length of the optical cavity (resonator) l! =N, n'L (where L is the resonator length of the laser, n'' is the effective refractive index, and N is an integer).

[Problem to be solved by the invention]

The present invention makes it possible to obtain an optical coupler for laser light using the above-mentioned prism while suppressing noise generation due to return light caused by shortening of the optical path length and reduction in sensitivity as a coupler, that is, reduction in coupling efficiency. .

[Means to solve the problem]

As shown in FIG. 1, the present invention provides an optical coupler for laser light using a prism (2).
The reflectance of the light incident surface (2a) is selected to be 25% or less.

[Effect]

As described above, by setting the reflectance of the light incident surface (2a) of the prism (2) to 25% or less, noise was reduced, that is, S/H was improved.

(Example) A case in which the optical coupler according to the present invention is applied to optical pickup will be described with reference to FIG.

That is, in this case, as mentioned above, an optical integrated circuit (optical I
This optical IC board (1) is placed on the optical IC board (1) having a
A prism (2) constituting an optical coupler is mounted while maintaining a required positional relationship with the photodiode PD formed in (1). Further, a semiconductor chip (3) is mounted on this optical IC substrate (1).

A semiconductor laser LD is mounted on the semiconductor chip (3), and the semiconductor chip (3) detects the light emitted from the semiconductor laser LD, for example, the light directed backward, and controls the power of the laser LD. For example, a photodiode PD for power monitoring with a PIN structure is formed. The light generated from the semiconductor laser diode LD is reflected by the light incident surface (2a) of the prism (2) and is transmitted through the lens system (4) to the optical disc (5) having bit information.
), and the reflected light modulated according to this information is introduced into the prism (2) from the light incident surface (2a) of the prism (2), and the multiple sets of photodiodes on the optical IC board (1) are The laser light is introduced into the PD, that is, coupled to the PD, thereby detecting the recorded information-modulated laser light and reading out the signal, as well as extracting servo signals such as a tracking servo signal and a focus servo signal.

Here, as the semiconductor laser LD, for example, a gain guide type so-called Cheebird stripe type semiconductor laser is used, which exhibits multimode oscillation that is not easily affected by noise with respect to returned light.

In this configuration, in the present invention, the reflectance R1 is set to 25% or less in order to give the light incidence surface (2a) of the prism (2) as the optical coupler a function as a half mirror. The reflectance R3 on the light incident surface (2a) can be selected by providing, for example, five to six layers of amorphous St and ZrO□ on this surface (2a) and controlling the thickness of each film. .

In this way, even if the optical path length from the emission end face of the semiconductor laser LD to the disk (5), so-called external cavity length L0□, is reduced to about 20IIIIIIl, the S
, /N does not decrease. That is, now the lens system (4)
By changing the focal length of L ext = 40mm5 L
ext ””30III11,■,. −=20mn
+ and the light incidence surface (2a) of each prism (2)
Curve (41) in FIG. 4 shows the results obtained by slightly changing the relationship between the reflectance R, and S, /N at each L ext, and measuring the worst value.

As shown in (42) and (43), no decrease in S and /N is observed when R is ≦25%. In other words, L
There is no inferiority compared to the case where ext is long.

In the above example, the present invention is used for optical pickup for an optical disc, but it can also be used as a coupler for various laser beams.

〔Effect of the invention〕

According to the optical coupler according to the present invention, as described above, even when the external oscillator length L ext of the semiconductor laser LD is as short as 20+mm in optical pickup, for example, the same effect can be obtained when L, □ is as long as about 40m+w. Since the instability of laser oscillation due to returned light, that is, the occurrence of sintering, can be avoided, and the reduction in S and /N can be suppressed, the optical pickup can be made smaller and lighter without deteriorating its characteristics. Therefore, it has extremely great practical benefits when applied to pickups in portable compact disc players and the like.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an example of an optical pickup to which the optical coupler according to the present invention is applied, Fig. 2 is a diagram showing the relationship between the external optical path length of a semiconductor laser and S/N, and Fig. 3 is a diagram showing the relationship between the reflectance of the prism and the signal. FIG. 4 is a diagram showing the relationship between the amount of light S0 and the amount of returned light St, and FIG. 4 is a diagram showing the relationship between the yellowing resistance and S, /N. (2) is a prism, (2a) is its light incident surface, and is a conductive laser. Zum's anti-LD is half agent Hidemori Matsukuma

Claims (1)

[Scope of Claims] An optical coupler for laser light using a prism, characterized in that the reflectance of the light incident surface of the prism is selected to be 25% or less.