Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
1998-09-18
2001-11-20
Davie, James W. (Department: 2881)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S034000, C372S050121, C372S102000
Reexamination Certificate
active
06320888
ABSTRACT:
This application is based on patent application Ser. No. 255,122/1997 filed Sep. 19, 1997 in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the structure of a frequency stabilized laser with mode hopping suppressed in an external cavity type frequency stabilized laser composed of an optically induced grating in an optical waveguide and a semiconductor LD (laser diode).
2. Description of the Related Art
A laser composed of an optically induced grating in a silica glass waveguide and a semiconductor LD is expected to find various uses as light sources for optical communication, optical information processing, optical measurement, and spectroscopy because of the following features: (1) It performs single-mode oscillation by utilizing the frequency selectivity of the grating. (2) Its temperature coefficient is smaller than that of a semiconductor laser. (3) Its oscillation frequencies can be controlled easily. (T. Tanaka, et al., Electron. Lett., vol. 32, No. 13, 1202 (1996); and Tanaka, et al., Presentations at the 1997 Congress of the Institute of Electronics, Information and Communication Engineers, C-3-160). The technique for fabrication of an optically induced grating was invented by Kenneth O. Hill, et al. (Japanese Patent Application Laying-open No. 7-140311(1995)). The optically induced grating will be referred to hereinbelow as a grating for simplification.
FIG. 13
is a schematic perspective view of a frequency stabilized laser produced by an earlier technology. In
FIG. 13
, the reference numeral
11
designates a semiconductor LD,
13
a core of a silica-based waveguide, and
14
a cladding of the silica-based waveguide. The reference numeral
15
denotes a grating,
16
an Si substrate, and
18
a portion, called a silicon terrace, which has been formed by removing silica glass for installation of the semiconductor LD.
The oscillation modes of the frequency stabilized laser composed of the grating in the silica-based waveguide and the semiconductor LD will be described. When an injection current is flowed into the semiconductor LD to cause light emission, only light of frequencies corresponding to the reflection spectrum of the grating is reflected by the grating. Thus, oscillation originates from a laser cavity which is a zone from the rear facet of the semiconductor LD to the grating. On the output endface of the semiconductor LD, an antireflection film to the interface with air is provided so that there will be no external feedback to the semiconductor LD except from the grating and the rear facet of the semiconductor LD. Also, the LD-side end face of the silica-based waveguide has a core-adjoining portion inclined with respect to a direction perpendicular to the optical axis of the core (see Japanese Patent Application Laying-open No. 6-230237 (1994)). Generally, the bandwidth of the reflection frequencies of a grating is about 50 GHz. On the other hand, the length of a laser cavity is about 0.5 cm. Thus, the frequency spacing of longitudinal modes is about 20 GHz. Since longitudinal modes exist with 20 GHz spacing in the 50 GHz bandwidth, about 3. longitudinal modes can be present. Of these longitudinal modes, only the one closest to the reflection center frequency of the grating is selected for oscillation. Generally, the reflectance of the grating is 40 to 99%, and the optical coupling loss between the semiconductor LD and the silica-based waveguide is about 4 dB±1.5 dB.
With the conventional frequency stabilized laser, however, the frequency of the longitudinal mode selected depends on temperature, causing a phenomenon in which the oscillating mode changes with a temperature change (to be called mode hopping). The reason will be explained as follows:
The temperature coefficient of the longitudinal modes of a conventional frequency stabilized laser is expressed in an approximated manner by the equation (1)
m
=
m
LD
n
LD
L
LD
+
m
WG
n
WG
L
WG
n
LD
L
LD
+
n
WG
L
WG
(
1
)
where
m
LD
and m
WG
are the temperature coefficient of the resonance frequencies of a resonator of a semiconductor LD, and the temperature coefficient of the resonance frequencies of a cavity prepared from a silica-based waveguide, respectively,
n
LD
and n
WG
are the equivalent index of a guide layer of the semiconductor LD, and the equivalent index of the silica-based waveguide, respectively, and
L
LD
and L
WG
are the cavity length of the semiconductor LD, and the silica-based waveguide length from the exit end of the semiconductor LD to the center of the grating, respectively. The grating is written into the silica-based waveguide, and the temperature coefficient of the reflection center frequency is equal to the temperature coefficient m
WG
of the silica-based waveguide. Since m
LD
≈10 m
WG
, the magnitude of the temperature coefficient m of the longitudinal modes is larger than the temperature coefficient m
WG
of the reflection center frequency of the grating.
That is, the temperature coefficient of the longitudinal modes does not equal the temperature coefficient of the reflection center frequency of the grating.
FIG. 14
is an explanatory drawing for mode hopping. Assume that oscillation is occurring in the Nth longitudinal mode. As a result of a change in temperature, the longitudinal mode closest to the reflection center frequency of the grating shifts to the (N+1)th, where oscillation takes place. That is, mode hopping occurs. In an example described in the paper T. Tanaka, et al., Electron. Lett., Vol. 32, No. 13, 1202 (1996), mode hopping occurred each time the temperature changed by 5° C. Since mode hopping increases the error rate of transmitted signals, realization of a method for suppressing it was desired.
The present invention has been accomplished to solve the above-described problem. Its object is to provide a frequency stabilized laser whose mode hopping due to a temperature change is suppressed by matching the temperature coefficient of longitudinal modes to the temperature coefficient of the reflection center frequency of a grating.
SUMMARY OF THE INVENTION
According to an aspect of the present invention for attaining the foregoing object, there is provided a frequency stabilized laser using an integrated external cavity and comprising a semiconductor laser diode installed on a substrate, an optical waveguide formed on the substrate, and an optically induced grating formed in the optical waveguide; wherein a material having a refractive index temperature coefficient opposite in sign to the refractive index temperature coefficient of the semiconductor laser diode is installed on a portion of the optical waveguide between the semiconductor laser diode and the optically induced grating, the portion being formed by removing an upper cladding and a core, or removing the upper cladding, the core, and a lower cladding.
Preferably, the optical waveguide is composed of silica glass. The portion formed by removing the upper cladding and the core, or removing the upper cladding, the core, and the lower cladding preferably crosses the waveguide at an angle of 80 to 90 degrees, more preferably 80 to 87 degrees.
Further preferably, the portion formed by removing the upper cladding and the core, or removing the upper cladding, the core, and the lower cladding is composed of a plurality of grooves. It is also preferred that a groove connecting the plurality of grooves together is present, and a liquid reservoir is connected to the connecting groove.
Preferably, the absolute value of the refractive index temperature coefficient of the material having the refractive index temperature coefficient opposite in sign to the refractive index temperature coefficient of the semiconductor laser diode is 1×10
−4
(1/K) or more, where K is the absolute temperature on the Kelvin scale.
The present invention also provides a method for producing the aforementioned frequency stabilized laser, including the steps of:
(a) obtaining a sili
Himeno Akira
Inoue Yasuyuki
Kaneko Akimasa
Takahashi Hiroshi
Tanaka Takuya
Davie James W.
Frank Robert J.
Nippon Telegraph & Telephone Corporation
Venable
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