JPH02212820A - Fiber type optical wavelength converter - Google Patents

Abstract

PURPOSE:To securely guide out parallel light which has a plane wave front by using a holographic element which is easily manufactured at low cost with superior mass-productivity by using the holographic element as a means which converts secondary light into parallel light, and controlling the temperature of a light source or the holographic element. CONSTITUTION:The laser light which is emitted by a semiconductor laser 11 is converted by a condenser lens 12 and made incident on a fiber type SHG (secondary higher harmonic generating element), its wavelength is converted, and the secondary light with a conic wave front which is the 2nd higher harmonic (SH wave) projected from the fiber type SHG10 is made incident on the holographic element 13. The holographic element 13 varies in complex transmissivity periodically with the distance (r) from the optical axis, so the incident secondary light with the conic wave front can be converted into the parallel light with the plane wave front. Thus, the holographic element 13 is used as a converting means to control the temperature of the light source or holographic element, thereby improving the mass-productivity and reducing the cost.

Description

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

BACKGROUND ART A tip pickup is known that uses an optical wavelength conversion element to convert the wavelength of a laser beam emitted from a laser light source into half, thereby making it possible to write and read information on and from a disk at a higher density. (Refer to Japanese Unexamined Patent Publication No. 61-50122).

As this optical wavelength conversion element, an optical fiber type SHG (Second Har
There is a tonics aenerator (second harmonic generating 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 polarized wave that generates Sl(wave) also propagates with the same phase velocity C/N(ω) (C: speed of light). This nonlinear polarized wave is guided at point A in the figure. Assume that an SH wave is generated in a direction that makes an angle of θ with the direction, and after a unit time, an SH wave is generated again in the θ direction at 8 points 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 for the SH wavelength of the cladding is nslam(2ω
), this phase matching condition is N (ω) swjl, , (2ω) cosθ with reference to the figure.
-----・-(1). That is, N ((IJ) < nglsd (2ω) =
As long as (2) is satisfied, SH waves are automatically generated in the θ direction with phase matching. In general, the refractive index of the cladding and core for the fundamental wave is nglsd(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 na + □ (ω) × N (ω・) × n (ω) ... (3)
It is. Also, considering the wavelength dispersion of the refractive index of the cladding, no, (ω) < nets□ (2ω), so ng
lsd (ω) <n (ω) <nglsd (2ω
)... If the condition (4) is satisfied, then equation (2) is satisfied for all fundamental modes no matter what the core diameter. Also, suppose nglsd(ω) < n glad (2ω)kn(
ω), a fundamental wave mode that satisfies equation (2) exists 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.

In order to use this SH wave as a light beam in, for example, an optical pickup for writing/reading information on an optical disk as described above, the light beam emitted from the fiber type SHG is focused as a beam spot on the information recording surface of the disk. There is a need. 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 Figure 9, fiber type 5HGIO
A conical prism 20 having a conical surface is arranged in the optical path after the beam exits, and the action of this conical prism 20 causes the S
If the H wave is collimated and converted into a parallel plane wave) and the conical equiphase surface is made into a flat surface, it becomes possible to condense 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.

Instead of the conical prism 20, it is conceivable to use a holographic element that is easy to manufacture, has excellent mass productivity, and can be obtained at low cost, as a conversion element that converts a conical equiphase surface into a flat surface.

This holographic element does not function properly if, for example, the apex angle θ of the conical wavefront of the SH wave differs from a designed value. Regarding this apex angle θ, a non-negligible error occurs due to variations in the shape and refractive index of the fiber type SHG. Therefore, by using a holographic element, the conical wavefront can be
In order to reliably convert the secondary light into parallel light with a plane wavefront, it is necessary to select and use a fiber type SHG that can obtain the desired apex angle θ.

SUMMARY OF THE INVENTION Therefore, the present invention provides a fiber-type optical wavelength conversion system that allows conical wavefront secondary light after wavelength conversion to be reliably derived as parallel light with a plane wavefront, regardless of variations in the shape and refractive index of the fiber-type SHG. The purpose is to provide equipment.

In the fiber type optical wavelength conversion device according to the present invention,
A holographic element is used as a means for converting the secondary light after wavelength conversion into parallel light, and the temperature of the light source or the holographic element can be controlled tilJ.

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

In FIG. 1 showing one embodiment of the present invention, laser light emitted from a semiconductor laser 11 as a light source is transmitted through a condensing lens 1.
2 and enters the fiber-type 5HGIO, where the wavelength is converted by the fiber-type 5HGIO. 2 of the conical wavefront, which is the SH wave emitted from the fiber type 5HGIO.
The secondary light enters the holographic element 13. As shown in FIGS. 2(a) and 2(b), the holographic element 13 has a plurality of annular zones 13b made of an opaque film centered on the optical axis at a constant pitch on one surface of a transparent substrate 13a made of glass or plastic. They are arranged in concentric circles. In this holographic element 13, since the complex transmittance changes periodically with respect to the distance r from the optical axis as shown in FIG. 3, a periodic amplitude change occurs with respect to the distance Mr for the incident wave. By appropriately setting the pitch of the annular zone 13b, the incident secondary light of a conical wavefront can be converted into parallel light of a flat wavefront.

In this way, by using the holographic element 13 as a means for converting secondary light into parallel light, the holographic element 13 is easier to manufacture and superior in mass production than conventionally used conical prisms. Therefore, the cost of the device can be reduced.

The semiconductor laser 11 is equipped with a temperature detector 14 such as a thermocouple or a thermistor to detect its temperature, and a temperature regulator 15 such as a bell or heater to adjust the temperature.
are installed on each. The temperature regulator 14 outputs a voltage according to the detected temperature, and this detected voltage becomes a comparison input of the error detector 16. As a reference input of the error detector 16, a set voltage output from the temperature setting device R17 according to the set temperature is supplied. The temperature setting by the temperature setting device 17 is performed, for example, by manual adjustment.

As the output of the error detector 16, a voltage corresponding to the error of the temperature detected by the temperature detector 14 with respect to the temperature set by the temperature setting device 17 is derived, and this error voltage is used to drive the temperature regulator 18 via the driver 18. It becomes input.

In such a configuration, as described above, if the apex angle θ of the conical wavefront of the incident light differs from the design value, the holographic element 13
does not function properly. Therefore, in order for the holographic element 13 to function correctly, it is necessary to make the cone apex angle θ equal to the design value. Here, when the wavelength of the incident light changes, the holographic element 13 has an almost equivalent effect even though the cone apex angle θ is changed in design, so the cone apex angle θ is designed by changing the wavelength of the incident light. can be equivalently equal to the value. On the other hand, it is known that the wavelength of the emitted laser light of the semiconductor laser 11 changes depending on its temperature.

Therefore, by adjusting the temperature of the semiconductor laser 11 and controlling the wavelength of the emitted laser beam, the fiber type S
It is possible to compensate for errors in the cone apex angle θ of the secondary light due to variations in the shape of the HG 1.0 and the refractive index, thereby ensuring that the secondary light with a conical wavefront can be converted into parallel light with a plane wavefront. The temperature of the semiconductor laser 11 is adjusted by manually adjusting the set temperature in the temperature setting device 17 until the emitted light from the holographic element 13 becomes parallel light. This is automatically performed by driving and controlling the temperature regulator 15.

In the above embodiment, as a means for converting secondary light into parallel light, a transparent substrate 13 having on one surface thereof a plurality of annular zones 13b made of opaque films arranged concentrically at a constant pitch is used.
We have described the case where a holographic element 13 consisting of a is used, but as shown in FIG.
8b1, that is, a transparent substrate 18a having unevenness on one surface
6(a), (
As shown in b), a transparent substrate 19 having ring zones 19b having a sawtooth cross section arranged concentrically at a constant pitch on one surface thereof.
A holographic element or the like consisting of a may be used, and the point is that the complex transmittance changes periodically with respect to the distance r from the optical axis, so that the incident wave has a periodic amplitude or phase with respect to the distance Mr. It suffices to have a configuration that causes a change in .

The phase characteristics of the holographic element shown in FIG. 4 cab, ib> are shown in FIG. 5, and the phase characteristics of the holographic element shown in FIG. 6 (ω, (i)) are shown in FIG.

Furthermore, in the above embodiment, a case has been described in which the cone apex angle θ of the incident light on the holographic element 13 is equivalently changed by adjusting the temperature of the semiconductor laser 11. Since changing the pitch of the holographic element 13b has almost the same effect as changing the cone apex angle θ in the design, it is also possible to configure the temperature of the holographic element 13 to be adjusted. In this case, the same configuration as in the above embodiment can be used, except that the temperature detector 14 and temperature regulator 15 are attached to the holographic element 13 instead of the semiconductor laser 11.

As described in detail of the invention, in the fiber type optical wavelength conversion device according to the present invention, a holographic element is used as a means for converting secondary light into parallel light, and the temperature of the light source or the holographic element is controlled. This structure compensates for errors in the cone apex angle θ of the secondary light caused by variations in the shape and bending of the fiber-type SHG, making it a holographic element that is easy to manufacture, has excellent mass productivity, and can be obtained at low cost. It is possible to reliably derive parallel light with a plane wavefront using .

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram showing an embodiment of the present invention, Fig. 2 is a plan view (ω and side sectional view) showing an example of a holographic element, and Fig. 3 is a transmission diagram of the holographic element shown in Fig. 2. Figure 4 is a plan view showing another example of the optical element (ω and side sectional view), Figure 5 is a diagram showing the phase characteristics of the holographic element in Figure 4, Figure 6 7 is a diagram showing the phase characteristics of the holographic element shown in FIG. 6, and FIG. 8 is a diagram showing a phase matching SHG using Cerenkov radiation method.
FIG. 9 is a schematic diagram showing a conventional example using a conical prism. Explanation of symbols of main parts 10... Fiber type optical wavelength conversion element 11...
... Semiconductor laser 13 ... Holographic element 14 ...
・Temperature detector 15...Temperature controller

Claims (4)

[Claims] (1) a light source, a fiber-type optical wavelength conversion element that converts the wavelength of emitted light from the light source as incident light, and a holographic element that converts the light wavelength-converted by the optical wavelength conversion element into parallel light; A fiber-type optical wavelength conversion device comprising: temperature control means for controlling the temperature of the light source. (2) The temperature control means includes a temperature detection means for detecting the temperature of the light source, a temperature setting means for setting the temperature, and an error in the temperature detected by the temperature detection means with respect to the temperature set by the temperature setting means. 2. The fiber type optical wavelength conversion device according to claim 1, comprising an error detection means and a temperature adjustment means for adjusting the temperature of the light source based on the error detected by the error detection means. (3) a light source, a fiber-type optical wavelength conversion element that converts the wavelength of emitted light from the light source as incident light, and a holographic element that converts the light wavelength-converted by the optical wavelength conversion element into parallel light; A fiber-type optical wavelength conversion device comprising: temperature control means for controlling the temperature of the graphic element. (4) The temperature control means includes a temperature detection means for detecting the temperature of the graphic element, a temperature setting means for setting the temperature, and an error of the temperature detected by the temperature detection means with respect to the temperature set by the temperature setting means. 4. The fiber type optical wavelength conversion device according to claim 3, further comprising: an error detecting means for detecting an error detected by the error detecting means; and a temperature adjusting means for controlling the temperature of the graphic element based on the error detected by the error detecting means.