Optical wavelength conversion module

An optical wavelength conversion module includes a semiconductor laser light source; a lens for focusing semiconductor laser light; and a fiber-type optical wavelength converter with a core and a clad having a refractive index lower than the core. At least one of the core and clad is formed of a non-linear optical material. The number of longitudinal modes of oscillation of the semiconductor laser light source is not less than three.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical wavelength conversion module 
comprising a fiber-type optical wavelength converter in which a 
fundamental wave of semiconductor laser light is incident to a core, and a 
second harmonic thereof is emitted as converted light. 
A non-linear optical effect is a phenomenon that when light is incident to 
a medium, there arises polarization proportional to higher order terms, 
quadratic and more, of an electric field of the light, and this phenomenon 
produces a second harmonic, a sum frequency, a difference frequency, and 
so on. 
The material producing such a phenomenon is called a non-linear optical 
material, and as the material, inorganic materials such as KH.sub.2 
PO.sub.4, LiNbO.sub.3, LiTaO.sub.3, etc. have been well known. Recently, 
however, organic materials represented by 2- methyl-4-nitrileaniline 
(MNA), 4-dimethylamino-3-acetoamidonitrobenzene (DAN), and 
3,5-dimethyl-1-(4-nitrophenyl)pyrazole (DMNP) has also attracted the 
attention because of their large non-linear optical constants. 
Recently, studies for applying such a non-linear optical material to a 
fiber-type optical wavelength converter for halving the wavelength from a 
semiconductor laser light source using inter-band transition of a 
semiconductor are performed eagerly, and converters using LiNbO.sub.3 or 
DMNP have been known. 
In such a converter, in order to produce a second harmonic or the like with 
a high efficiency, it is important to design the converter to confine a 
fundamental wave with a high energy density, and ensure the interaction 
length between the fundamental wave and the higher harmonic. Therefore, 
either a core or a clad of an optical fiber is formed of single crystal or 
polycrystal of a non-linear optical material, and an amorphous material 
such as glass is used as either the clad or the core, so that a 
fundamental wave is guided into the core to thereby obtain a high 
conversion efficiency. FIG. 3 shows the state where a fundamental wave 6 
having passed a core 41 of a fiber-type optical wavelength converter 4 is 
emitted after converted into a second harmonic 5. 
In a fiber-type optical wavelength converter, it is also necessary to make 
the propagation rate of a fundamental wave coincident with that of a 
produced second harmonic, that is, to make phase matching between the 
fundamental wave and the second harmonic. The phase matching implicates 
that, as shown in FIG. 4, on the assumption that a second harmonic is 
produced at a point A from light propagating through the core 41 and leaks 
out to a clad 42 at an angle .alpha., in the case where the direction 
.alpha. at a point B after elaspe of a unit time is coincident to the 
equiphase plane of the first-mentioned second harmonic, a second harmonic 
is radiated (Cherenkov Radiation) in the direction of this angle .alpha.. 
Let the refractive index of the clad 42 to a fundamental wave be n.sub.S 
(.omega.), the refractive index of the core 41 be n.sub.G (.omega.), and 
the refractive index of the clad 42 to a second harmonic be n.sub.S 
(2.omega.), phase matching is made automatically to thereby make Cherenkov 
Radiation possible only if the following condition is satisfied. 
EQU n.sub.S (2.omega.)&gt;n.sub.G (.omega.)&gt;n.sub.S (.omega.) 
For example, the investigation to use an optical wavelength conversion 
module comprising such a fiber-type optical wavelength converter for 
reading an optical disk is being advanced. To this end, an optical disk 
light source for generating a fundamental wave must be small, and 
therefore a semiconductor laser is employed. 
In this case, noises of semiconductor laser light is amplified because the 
intensity of a second harmonic is proportional to the square of that of a 
fundamental wave, so that a high S/N ratio is required in the 
semiconductor laser light. 
In the case of multi-longitudinal modes (spectra) of a semiconductor laser 
light, a sum frequency wave of two longitudinal modes is produced together 
with a second harmonic in a fiber-type optical wavelength converter, and 
the intensity of the sum frequency wave fluctuates as time goes. It has 
been therefore considered that it is preferable and necessary to use a 
single longitudinal mode of semiconductor laser oscillation in order to 
obtain light of narrow wavelength range. 
If the number of longitudinal modes of semiconductor laser light is made 
one, however, when the above-mentioned semiconductor laser light is 
irradiated to a fiber-type optical wavelength converter, the semiconductor 
laser light source is badly affected by so-called "return" of light which 
is a phenomenon that the reflected light returns to the semiconductor 
laser light source. It is said that the effect appears not only in the 
case of strong reflected light fed back to a laser aperture, but also in 
the case of very weak one about 10.sup.-7 of light emitted from the 
semiconductor laser. The distance between the semiconductor laser light 
source and a reflection point for the effect to appear is in a wide range 
from a .mu.m order to a km order ("O plus E", Feb. 1984, pp.89 to 97). 
Mode jump is induced in the semiconductor laser by this return phenomenon, 
and many noise components are produced in output light. Consequently the 
S/N ratio of a second harmonic outputted from the fiber-type optical 
wavelength converter deteriorates. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to solve the foregoing 
technical problems, and to provide an optical wavelength conversion module 
which is hardly subjected to the effect of "return", and which is able to 
realize a superior S/N ratio. 
In an optical wavelength conversion module of the present invention, the 
number of longitudinal modes of a semiconductor laser light is not smaller 
than three. 
By making the number of longitudinal modes of a semiconductor laser not 
smaller than three, possible interference of semiconductor laser light is 
remarkably reduced, so that mode jump hardly occurs. Further, since all 
the modes do not jump even if mode jump occurs, noises caused by mode jump 
is reduced in comparison with those in a single-mode semiconductor laser. 
Further, because of multi-longitudinal modes, not only second harmonics of 
respective modes but also a sum frequency wave of any two modes are mixed 
in the output from the fiber-type optical wavelength converter. Therefore, 
the intensity of the sum frequency wave fluctuates if the intensity of the 
modes fluctuate. By making the number of modes not smaller than three, 
however, it is possible to reduce the fluctuation in output ox the sum 
frequency wave and the second harmonic caused by the fluctuation of the 
respective modes.

DETAILED DESCRIPTION OF THE INVENTION 
Preferred embodiments of the present invention will be described in detail 
with reference to the drawings. 
FIG. 1 is a conceptual diagram illustrating a basic configuration of an 
optical wavelength conversion module according to an embodiment of the 
present invention. In the optical wavelength conversion module, 
semiconductor laser light generated from a semiconductor laser light 
source 1 is collimated by a spherical lens 2, the collimated light is 
focused by a spherical lens 3, and the focused light is made incident to a 
fiber-type optical wavelength converter 4. In order to minimize the lens 
diameter, the aperture angle of the spherical lens 3 is designed so as to 
be substantially equal to that of the fiber-type optical wavelength 
converter 4. 
The fiber-type optical wavelength converter 4 includes a core 41 and a clad 
42. 
A well-known non-linear optical material such as MNA, DAN, or the like, is 
applied to either one of or both of the core 41 and the clad 42. 
If semiconductor laser light is incident to the core 41 of the fiber-type 
optical wavelength converter 4, Cherenkov Radiation of the semiconductor 
laser light as a fundamental wave is produced from the out-going end 
surface of the fiber-type optical wavelength converter 4, and a produced 
second harmonic (converted light) 5 is emitted so as to be radiated while 
being diffused in a ring shape. The emitted converted light is made to 
pass through a wavelength filter (not shown) for cutting off a fundamental 
wave, and collimated by a lens system (not shown) so as to be used for 
reading an optical disk. 
Next, an example of producing a fiber-type optical wavelength converter 
will be described. 
Experimental Example 1 
A capillary tube of SF1 glass (made by HOYA GLASS CORP.) having an inner 
diameter of 1.6 .mu.m, an outer diameter of 1.0 mm, and a length of 50 mm 
was filled with melted DAN, and then, the tube was cooled to grow up 
single crystal as a core to thereby prepare a fiber-type optical 
wavelength converter. The method of growing up the core was a method in 
which a crystal is grown up from an end by Bridgeman Method after the 
melted DAN is drawn up by use of a capillary tube phenomenon (with respect 
to the detail of the method, reference is made to Japanese Unexamined 
Patent Publication No. Hei-3-111826). 
This fiber-type optical wavelength converter was cut out in the length of 
10 mm so that its end surface was perpendicular to a light axis, and the 
optical wavelength converter was set so as to be in parallel to the light 
axis. 
When semiconductor laser light (the number of longitudinal modes and the 
output being 10 and 100 mW respectively) of a wavelength 980 nm was 
incident to the core of the optical wavelength converter, the converted 
light of wavelength 490 nm was emitted from the fiber out-going end to the 
air. 
When the relative intensity noise RIN (reference is made to "O plus E", 
Feb. 1984, pp. 95) of the obtained converted light which was expressed by 
EQU RIN=&lt;.DELTA.P.sup.2 &gt;/P.sup.2 .DELTA.f 
was measured, the obtained value was 10.sup.-10, in which .DELTA.f 
represents a unit frequency band width, &lt;.DELTA.P.sup.1 &gt; represents a 
square average value of noise intensity in the unit frequency band width, 
and P represents light intensity of all frequency bands. 
Next, a fiber-type optical wavelength converter was made up by use of DMNP 
in the same manner. 
When semiconductor laser light (the number of longitudinal modes and the 
output being 10 and 100 mW respectively) of the wavelength 900 nm was 
incident to the core of this optical wavelength converter, the converted 
light of wavelength 450 nm was emitted from the out-going end of the fiber 
to the air. 
When the relative intensity noise RIN of the obtained second harmonic was 
measured, the obtained value was 10.sup.-11. 
Experimental Example 2 
When semiconductor laser light (the output 100 mW) of a single mode was 
incident to the above-mentioned fiber-type optical wavelength converters 
using DAN and DMNP respectively, measured values of the relative intensity 
noise RIN of obtained converted lights were 10.sup.-8 and 10.sup.-9 
respectively. 
As is apparent from the above experimental examples, it is possible to 
reduce noises to a large extent in comparison with the case of a single 
mode by increasing the number of modes of semiconductor laser light. 
The conventional technique of coating the end surface of a fiber-type 
optical wavelength converter with a reflection protection film may be 
combined with the above embodiment. Moreover as shown in FIG. 2, a 1/2 
wavelength plate 51 may be inserted, or a prism beam expander 52 may be 
inserted for changing the beam cross-section of the semiconductor laser 
light source 1 from an ellipse to a circle. Further, various modifications 
of design may be provided within the scope of the present invention. 
As has been described, according to the present invention, by making the 
number of longitudinal modes of a semiconductor laser not smaller than 
three, it is possible to realize an optical wavelength conversion module 
eliminating the effects of noises or output fluctuations and having a 
superior S/N ratio.