Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2000-12-01
2004-05-18
Porta, David (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S022000, C372S029014, C372S029015
Reexamination Certificate
active
06738398
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a Second Harmonic Generation (otherwise known as “SHG”) laser light source whose optical output signal can be modulated using a simple circuit and to a method of modulation for use therewith.
2. Description of the Prior Art
The surface recording density of optical disks has increased dramatically as the recording capacity of the disk has increased. This trend has led to a smaller on disk spot where the beam is focused. Accordingly, there is increased demand for the wavelength of the light source used to write and read from the disk to be as short as possible. The light sources for producing short wavelength beams include the SHG laser light source. Japanese unexamined patent application 11-26861 discloses an SHG light source which injects a fundamental wave into a nonlinear optical crystal to produce a second harmonic wave, which is a short wavelength coherent light. This type of system is shown in FIG.
1
.
In
FIG. 1
, a distributed Bragg reflection (also known as “DBR”) laser diode
1
produces a fundamental wave. An SHG device
2
is supplied with an input of the fundamental wave, converts the wavelength thereof, and outputs a second harmonic wave. An external modulator
3
is supplied with the second harmonic wave from the SHG device
2
, and modulates the wave, and then outputs a modulated wave.
The
FIG. 1
system operates as follows. Drive circuit
4
drives DBR laser diode
1
to output a fundamental wave which is supplied to SHG device
2
, which converts the wavelength thereof, and outputs a second harmonic wave. The second harmonic wave is modulated or attenuated by external modulator
3
so that the power of the wave is controlled. As described in 11-26861, SHG device
2
has such a relationship between the output power of the second harmonic wave and the input wavelength as shown in FIG.
2
. As shown in
FIG. 2
, the output power drastically drops once the input wavelength deviates from a specific value. Accordingly, the output power is heavily dependent on the wavelength of the fundamental wave. Also, the nature of the DBR laser diode
1
is such that the wavelength of the fundamental wave also changes when the output power thereof is changed by altering the drive current.
Consequently, even when the drive current of DBR laser diode
1
is increased to raise the power of the fundamental wave, for example, in an attempt to boost the power of SHG laser light source, the wavelength of the fundamental wave may shift. The shift would result in a drop in the output power of the SHG laser light source. Accordingly, in the prior art system, it is essential that the power of the second harmonic wave outputted by the SHG device be constant and the optical output of the SHG laser light source be changed by an external modulator
3
, in order to suitably modulate the optical output. Consequently, the prior art system is always provided with an external modulator
3
. The external modulator
3
, in turn, requires an optical system which couples the modulator to the SHG device
2
. This requirement hinders use of the prior art system as a pickup device for optical disks which is increasingly required to be smaller.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to overcome the aforementioned and other disadvantages, deficiencies and problems of the prior art.
Another object is to provide an SHG laser light source whose optical output can be modulated using a simple mechanism without having to provide an external modulator.
A further object is to provide a method of modulation for use with the SHG laser light source.
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Hirata Takaaki
Iio Shinji
Inoue Takeshi
Kitaoka Yasuo
Yamamoto Kazuhisa
Monbleau Davienne
Moonray Kojima
Porta David
Yokogawa Electric Corporation
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