Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
2001-02-09
2003-09-02
Leung, Quyen (Department: 2828)
Coherent light generators
Particular component circuitry
For driving or controlling laser
Reexamination Certificate
active
06614820
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor laser driving method and an optical disc apparatus, more particularly, to a semiconductor laser driving method for driving a semiconductor laser with a RF-modulated driving current and an optical disc apparatus using the driving method for driving a semiconductor laser used as a light source.
2. Description of the Related Art
As to semiconductor lasers used as light sources of optical pick-up devices in optical disc apparatuses, there are cases where a semiconductor laser is driven by a driving current made by modulating a D.C. current with a high-frequency (RF) current, that is, RF modulation of a semiconductor laser is done, for the purpose of reducing so-called optical feedback noise. In such cases, conventional technologies were configured to optimize the RF-modulating frequency, amplitude and/or waveform, depending on the optical system in which the semiconductor laser is used.
According to recent researches by the Inventor, in optical disc apparatuses used for reproduction of DVD-ROM (digital video disc ROM) with a much higher recording density than CD-ROM (compact disc ROM), a serious increase in jitter, probably caused by noise, was confirmed to occur at a specific optical output, regardless of sufficient RF modulation of an AlGaInP semiconductor laser used as a light source of an optical pick-up. Since such an abnormal increase in jitter invites deterioration of reading characteristics, a countermeasure therefor is desired.
This problem appeared in a process of reproduction of DVD-ROM with a high recording density. Essentially, however, it is presumably caused by the decreased pit sizes along with progressively high recording densities of optical discs. Taking an optical disc with the diameter of 12 cm, for example, the problem becomes apparent when its capacity is on the order of gigabytes. For example, the capacitor of an optical disc with the diameter of 12 cm for use with a semiconductor laser with the oscillation frequency of 650 nm (DVD-ROM, or the like) is 4.7 gigabytes, and the capacity of an optical disc with the diameter of 12 cm for use with a semiconductor laser with the oscillation frequency of 780 nm is 0.64 gigabytes (640 megabytes).
OBJECT AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a semiconductor laser driving method which enables the use of a semiconductor laser under a low noise by preventing an abnormal increase in jitter at a specific optical output when the semiconductor laser is RF-modulated, and to provide an optical disc apparatus using the driving method.
The Inventor made active researches to overcome the above-indicated problems involved in the conventional techniques, as summarized below.
Here are repeated the problems involved in the conventional techniques in greater detail. The Inventor found an abnormal increase in jitter at a specific RF level, namely, at approximately 1.3 V (corresponding to the optical output of about 2.5 mW), in the dependency of jitter upon the RF level (linearly responding to the optical output of the semiconductor laser) obtained upon RF modulation of 350 MHz to an index-guided AlGaInP semiconductor laser with the oscillation frequency of 650 nm used as a light source of the optical pickup in a DVD-ROM optical disc apparatus. This phenomenon appeared under a condition with sufficient RF modulation and always appeared at the same RF level. Therefore, it was not a phenomenon caused by an optical feedback noise. Additionally, the same phenomenon was found also in various types of DVD semiconductor lasers.
FIG. 1
shows typical characteristics of optical output (L) to driving current (I) in both a case with RF modulation and a case without RF modulation. It is known from
FIG. 1
that, without RF modulation, namely, in the D.C. driving mode, linear L-I characteristics appear; however, with RF modulation, the threshold current I
th
decreases, and non-linear undulation occurs in the L-I characteristics. In terms of changes in differential quantum efficiency, the undulation is periodically attenuating fluctuation. Such undulation in L-I characteristics have not been understood heretofore.
The Inventor also found that the intrinsic noise (quantum noise) changed like “lumps” in substantial synchronism with undulation in the L-I characteristics. That is, although typical characteristics of relataive intensity noise (RIN) to average optical output (Pout) of a RF-modulated semiconductor laser are as shown in
FIG. 2
, the RIN-to-Pout characteristics shown here represent “lump”-like changes in the intrinsic noise approximately synchronizing with the undulation in the L-I characteristics. In
FIG. 2
, P
1
, P
2
, P
3
, . . . are average optical outputs in which RIN is maximized.
Then, as a result of observation of optical waveforms responsive to RF-modulation, it was found that a first pulse, second pulse and third pulse of relaxation oscillation occurred in synchronism with undulation in L-I characteristics. The start positions of respective pulses substantially coincide with the minimum positions (bottoms) in the L-I characteristics curve, and intrinsic noise derives from occurrence of a new oscillation mode near here.
FIG. 3
shows the aspect of this phenomenon. In
FIG. 3
, D
1
and D
2
are bottoms in the L-I characteristics curve. Generation of intrinsic noise caused by occurrence of the new oscillation mode is principally a phenomenon similar to an increase in quantum noise caused by occurrence of kinks, and can be regarded as the origin of lumps in the intrinsic noise.
As explained above, it has been confirmed RF modulation of a semiconductor laser invites the phenomenon that the intrinsic noise periodically exhibits peaks at a specific optical output and that RF modulation of a semiconductor laser used as the light source of the optical pickup in an optical disc apparatus causes the problem of increasing the jitter at these peaks. The nature of the phenomenon can be briefed as follows.
Positions of lumps (positions of optical outputs) in the intrinsic noise in RIN-to-Pout characteristics are determined by the relation between relaxation oscillation frequency (fr) of the semiconductor laser and RF-modulation frequency (RF modulation cycle), RF-modulation waveform, RF-modulation amplitude, and so forth, and have the following natures.
(Nature 1) The larger the RF-modulation amplitude, the longer the cycle of changes in “lump” positions in the intrinsic noise.
(Nature 2) The higher the RF-modulation frequency (the shorter the RF-modulation cycle), the longer the cycle of changes in “lump” positions in the intrinsic noise.
(Nature 3) The higher the relaxation oscillation frequency, the shorter the cycle of changes in “lump” positions in the intrinsic noise.
(Nature 4) As the temperature becomes higher, the cycle of changes in “lump” positions in the intrinsic noise becomes slightly longer.
(Nature 5) When the feedback light increases, the noise amount increases and the cycle of changes in “lump” positions in the intrinsic noise becomes shorter.
(Nature 6) When the RF-modulation frequency becomes higher, the noise amount during low optical output decreases.
These natures derive from the following reasons.
FIGS. 4A
to
4
C show changes in optical response to changes in D.C. bias current value in which the D.C. bias current value increases from
FIG. 4A
to
FIG. 4B
to FIG.
4
C. When the D.C. bias current value is low (FIG.
4
A), which represents the status where only the first peak of relaxation oscillation is output, the first peak of relaxation oscillation grows in response to the D.C. bias current. When the D.C. bias current value increases to represent the status shown in
FIG. 4B
where the effective pulse width Wp overlaps excitation of the second peak of relaxation oscillation, the second peak of relaxation oscillation starts to generate. It is known that a semiconductor laser, in general, generates intrinsic noise (quantum noise originating from spontaneous radiation) when
Aga Keigo
Hirata Shoji
Leung Quyen
Sonnenschein Nath & Rosenthal
Sony Corporation
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