JPH02205828A - Fiber type light wavelength converting device - Google Patents

Abstract

PURPOSE:To obtain the fiber type light wavelength converting device which is easily manufactured and superior in mass-productivity by using a holographic element which varies in complex transmissivity periodically with the distance from the optical axis as a means which converting liquid into parallel light. CONSTITUTION:Secondary light which has a conic wave front as an SH wave emitted by a fiber type SHG10 is made incident on the holographic element 30. The holographic element 30 has rings 32 made of opaque films on one surface of a transparent substrate 31 of glass or plastic concentrically at constant pitch around the optical axis. This holographic element 30 varies in complex transmissivity periodically with the distance (r) from the optical axis, as to pitch of the rings 32 is set properly to convert the incident secondary liquid with the conic wave front into parallel light having a lane equiphase plane. Thus, the holographic element 30 is used as the means for converting the secondary light into the parallel light to facilitate the manufacture, improve the mass-productivity, and reduce the cost of the device.

Description

[Detailed description of the invention] Technical field The present invention relates to a fiber type optical wavelength conversion device.

BACKGROUND ART An optical pickup is known in which the wavelength of a laser beam emitted from a laser light source is halved using an optical wavelength conversion element, thereby making it possible to write and read information to and from a disk at higher density. (Refer to Japanese Unexamined Patent Publication No. 61-50122).

As this optical wavelength conversion element, an optical fiber type SHG (Second Mar
sonlcs Generator (second harmonic generation element).

The optical fiber type SHG employs phase matching based on the Cerenkov radiation method. In this method, the second harmonic (hereinafter abbreviated as SH wave) is automatically phase-matched.
is possible. FIG. 8 is a conceptual diagram thereof.

In FIG. 8(a), the fundamental mode has an effective refractive index N(
ω), the nonlinear polarization wave that generates the SH wave also has the same phase velocity C/N(ω) (C: speed of light)
propagate with Assume that this nonlinear polarization wave generates an SH wave at point A in the figure in a direction that makes an angle of θ with the waveguide direction, and after a unit time, it generates an SH wave again in the θ direction at point B as before. .

For example, if the SH wave generated at point A propagates through the cladding and reaches point C after a unit time, and θ is an angle such that AC and BC are orthogonal, then the nonlinear polarization wave will cause the SH wave generated between AB to reach point C. The wavefront becomes BC, and in the end, a coherent SH wave is generated.

The refractive index of the cladding for the SH wavelength is n. 1□ (2ω)
Then, this phase matching condition is N (ω) ”nc+y (2(IJ) eO8θ
------・(1). That is, N (ω) < n −Iha (2ω) −
-(2), the SH wave is automatically generated in the θ direction with phase matching. In general, the refractive index of the cladding and core for the fundamental wave is nclad(ω
) and n(ω), and assuming that the overlayer is air, the conditions for the fundamental wave to propagate in the core as a mode are ``clan(ω)×N(ω)×n(ω)...(3)
It is. Also, considering the wavelength dispersion of the refractive index of the cladding, n clad (ω) < n clad (2ω), so n glad ((IJ) < n ((L))
< n elmd (2CIJ)...If the condition (4) is satisfied, then the equation (2) is satisfied for all fundamental modes no matter what the core diameter. Also, suppose nclad ((IJ) < nclad (2ω) <
n ((IJ), there is a fundamental mode that satisfies equation (2) within a certain range of film thickness.

The SH waves generated in this way propagate as a cladding mode that undergoes repeated total reflection at the boundary between the cladding and air, as shown in Figure 8(b), and form a conical shape from one end of the fiber in the direction determined by the angle θ. It is emitted. Further, the equiphase front of the output wavefront of the SH wave outputted in this manner has a conical shape with the central axis of the fiber as its axis.

For example, in order to use this SH wave as a light beam in an optical pickup for writing/reading information on an optical disk as described above, it is necessary to use a core '0 <- type sHG.
It is necessary to focus the emitted light beam on the information recording surface of the disk as a beam spot. However, even when condensing the emitted light beam from a fiber-type SHG, since the equiphase plane of the emitted light is conical, it is difficult to condense the light to the diffraction limit using a conventional condensing lens made of a spherical or aspherical lens. It is impossible to do so.

Therefore, as shown in Fig. 9, fiber type 5HG10
A conical prism 20 having a conical surface is disposed in the optical path after the beam exits, and by the action of this conical prism 20, S
By collimating the H wave (converting it into a parallel plane wave) and making the conical equiphase surface plane, it becomes possible to condense the light up to the diffraction limit using a conventional condensing lens.

However, since the conical prism 20 has a conical shape, it is difficult to manufacture and has poor mass productivity, which is disadvantageous in terms of cost reduction.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a fiber-type optical wavelength conversion device that can collimate SH waves using an optical element that is easy to manufacture, has excellent mass productivity, and can be obtained at low cost.

In the fiber type optical wavelength conversion device according to the present invention,
As a means for converting the light wavelength-converted by the fiber-type SHG into parallel light, a holographic element whose complex transmittance changes periodically with respect to the distance from the optical axis is used.

Example Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

In FIG. 1 showing an embodiment of the present invention, two conical wavefronts, which are SH waves emitted from a fiber type 5HGIO, are shown.
The secondary light is incident on the holographic element 30. The holographic element 30, as shown in FIG. 2 (ω, mountain),
A plurality of annular zones 32 made of an opaque film are arranged concentrically around the optical axis at a constant pitch on one surface of a transparent substrate 31 made of glass or plastic. In this holographic element 30, since the complex transmittance changes periodically with respect to the distance i from the optical axis as shown in FIG. 3, a periodic amplitude change occurs with respect to the distance r for the incident wave. By appropriately setting the pitch of the annular zone 32, the incident secondary light having a conical wavefront can be converted into parallel light having a flat equiphase surface.

In this way, by using the holographic element 30 as a means for converting secondary light into parallel light, the holographic element 30 is easier to manufacture and superior in mass production than conventionally used conical prisms. Therefore, the cost of the device can be reduced.

In the above embodiment, as a means for converting secondary light into parallel light, a holographic element consisting of a transparent substrate 31 having on one surface thereof a plurality of annular zones 32 consisting of opaque films arranged concentrically at a constant pitch is used. 30, as shown in FIG. 4(a), φ), a plurality of annular zones 33 with a rectangular cross section arranged concentrically at a constant pitch.
That is, a holographic element consisting of a transparent substrate 34 having unevenness on one surface, and an annular zone 35 having a sawtooth cross section arranged concentrically at a constant pitch as shown in FIG. 6 (ω, mountain) on one surface. A holographic element or the like consisting of a transparent substrate 36 having a transparent substrate 36 may be used.The point is that the complex transmittance changes periodically with respect to the distance r from the optical axis. Any configuration that can cause a change in amplitude or phase is sufficient.

FIG. 5 shows the phase characteristics of the holographic element in FIG. 4 (ω9 song), and FIG. 7 shows the phase characteristics of the holographic element in FIG. 6(a) (mountain).

As described in detail of the invention, in the fiber-type optical wavelength conversion device according to the present invention, as a means for converting secondary light into parallel light, the complex transmittance changes periodically with respect to the distance from the optical axis. It has a configuration using holographic elements,
This holographic element is easier to manufacture than conventionally used conical prisms, and the so-called relief type with an uneven surface can be pressed, making it suitable for mass production. This will result in lower costs.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing one embodiment of the present invention, Fig. 2 is a plan view (a) and side sectional view showing an example of a holographic element), and Fig. 3 is a block diagram of the holographic element shown in Fig. 2. A diagram showing the transmittance characteristics, FIG. 4 is a plan view (ω and side sectional view) showing another example of the optical element, FIG. 5 is a diagram showing the phase characteristics of the holographic element in FIG. 4, and FIG. The figure shows a plan view (a) and a side sectional view (b) showing still another example of a holographic element, Fig. 7 shows a phase characteristic of the holographic element shown in Fig. 6, and Fig. 8 shows a Cherenkov radiation method. A conceptual diagram of a phase matching SHG, FIG. 9 is a configuration diagram showing a conventional example using a conical prism. Explanation of symbols of main parts 10... Fiber type optical wavelength conversion element 20...
... Conical prism 30 ... Holographic element 32.33.3
5... Ring applicant Pioneer Corporation

Claims (4)

[Claims] (1) A fiber-type optical wavelength conversion element that converts the wavelength of incident light, and a conversion means that converts the light wavelength-converted by the optical wavelength conversion element into parallel light, and the conversion means has a complex transmittance. A fiber-type optical wavelength conversion device characterized by using a holographic element that changes periodically with respect to the distance from the axis. (2) The fiber-type optical wavelength conversion according to claim 1, wherein the holographic element is made of a transparent substrate having on one surface thereof a plurality of annular zones made of opaque films arranged concentrically at a constant pitch. Device. (3) The fiber-type optical wavelength conversion according to claim 1, wherein the holographic element comprises a transparent substrate having on one surface thereof a plurality of annular zones having a rectangular cross section arranged concentrically at a constant pitch. Device. (4) The fiber-type optical wavelength conversion according to claim 1, wherein the holographic element comprises a transparent substrate having on one surface thereof a plurality of annular zones having a sawtooth cross section arranged concentrically at a constant pitch. Device.