JPH0722218B2 - Frequency stabilized light source - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a frequency-stabilized light source for optical communication, optical measurement, and optical recording.

2. Description of the Related Art Conventionally, optical feedback using a diffraction grating has been performed to stabilize the frequency of a semiconductor laser device, and an echelette with mechanical engraving has been used as these diffraction gratings. The Echelette grating has a sawtooth-shaped grating groove, and the diffraction efficiency is high with respect to the blaze angle and the blaze wavelength determined by the tilt of the grating. Therefore, the light emitted from the semiconductor laser element is efficiently reflected by the diffraction grating and is strongly coupled again to the original semiconductor laser element, so that the oscillation mode of the semiconductor laser element is unified and the frequency is stabilized. The oscillation wavelength is geometrically determined by the positional relationship between the diffraction grating and the semiconductor laser device. FIG. 4 shows a conventional embodiment. The light emitted from the semiconductor laser device 41 becomes parallel light from the lens 42 and enters the echelette grating 43. The incident light is dispersed according to its wavelength, only a specific wavelength is coupled with the semiconductor laser element 41, and the semiconductor laser element 41 oscillates at that wavelength. In particular, when the incident surface and diffracted light have a specular reflection relationship with the groove surface of the Echelette grating 43, the diffraction efficiency is increased, the coupling with the semiconductor laser element 41 is strengthened, and oscillation is stabilized. (For example, Electronics Letter (ELECTRONIC
(S LETTER) Volume 21, No. 15, Page 658, 1985) Problems to be Solved by the Invention However, since the Echelette grating used for the frequency-stabilized light source described above is mainly manufactured by machine engraving, groove pitch error may occur. The generated ghost and stray light cause the wavelength of the semiconductor laser device to become unstable and generate noise. The setting of the lattice constant is also restricted by the screw pitch. This imposes restrictions on the design of the optical system including the lens. In addition, the machine engraved echelette lattice takes time to manufacture, and the ruling engine for engraved lines uses the conventional echelette lattice because the echelette lattice created for ultra-precision machines becomes very expensive. Frequency stabilized light sources are expensive. There is a method of mass-producing the Echelette lattice to make a replica from the master lattice. As shown in FIGS. 5 (a) and 5 (b), the tip portion becomes flat or a whisker is formed. For this reason, when a replica is used to transfer the shape, the characteristics may be different from the original, or the replica may not be obtained.

In view of the above problems, the present invention provides a frequency stabilized light source that is low in cost and low in noise.

Means for Solving the Problems In order to solve the above problems, the frequency-stabilized light source of the present invention uses a holographic diffraction grating to perform optical feedback.

Effect The present invention provides a frequency-stabilized light source with low cost and low noise by the above-mentioned configuration.

Embodiment A frequency-stabilized light source according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a block diagram of a frequency stabilizing light source in an embodiment of the present invention. Reference numeral 1 is a semiconductor laser element, 2 is a lens, 4 is a holographic diffraction grating, and 5 is an antireflection film. Light emitted from the semiconductor laser element 1 is reflected by the lens 2
It becomes parallel light and enters the holographic diffraction grating 4. The incident light is dispersed according to the wavelength, only a specific wavelength is coupled with the semiconductor laser device 1, and the semiconductor laser device 1 oscillates at that wavelength.

The diffraction efficiency of the holographic diffraction grating depends on the polarization direction of incident light and the ratio λ / d of the grating constant d and the wavelength λ. This is shown in FIG. Those having an electric vector in a direction parallel to the groove direction of the holographic diffraction grating are P-polarized light, and those perpendicular to the same are S-polarized light. The efficiency is high for a specific λ / d for P or S polarization. Since the wavelength width of the output light of the semiconductor laser device 1 is only a few angstroms, if the d of the diffraction grating is set to be high in efficiency, the intensity of the diffracted light becomes high and a strong optical feedback is applied to the semiconductor laser device 1. Since the output light of the semiconductor laser device 1 is polarized in the horizontal direction with respect to the joint surface, by arranging the joint surface of the semiconductor laser device 1 horizontally or vertically with respect to the groove direction of the holographic diffraction grating 4. It can be easily constructed in the P or S polarization state. The diffraction efficiency is particularly high in the range where λ / d is 0.2 <λ / d <1.7 with respect to the incident light in the P or S polarization state.

The holographic diffraction grating can be easily prepared by exposing a photoresist to two light fluxes having a single wavelength and developing the photoresist. In addition, the shape of the holographic grating is smooth and symmetrical, making it easy to transfer the shape,
The original groove structure is directly transferred. Therefore, the transferred replica has the characteristics of the original holographic diffraction grating. By making a replica, holographic diffraction gratings having the same characteristics can be easily produced in large quantities. Therefore, it is possible to construct a frequency-stabilized light source at low cost by using the holographic diffraction grating. Further, since the holographic grating formed by the two-beam interference exposure method has less ghost due to the pitch error than the Echelette grating by mechanical engraving, even if optical feedback to the semiconductor laser element 1 is performed, an unstable coupling state does not occur. Have an advantage.

FIG. 3 is a configuration diagram of a frequency stabilized light source showing a second embodiment of the present invention. The output light from the semiconductor laser device 31 is dispersed by the holographic concave diffraction grating 35, and the light of a specific wavelength is coupled to the semiconductor laser device 31 again. By making the diffraction grating concave, no lens is required.

EFFECTS OF THE INVENTION As described above, the present invention provides an inexpensive and noise-free frequency-stabilized light source by using a semiconductor laser device and a holographic diffraction grating of 0.2 <λ / d <1.7 for a wavelength λ and a lattice constant d. Can be provided.

[Brief description of drawings]

FIG. 1 is a block diagram of a frequency-stabilized light source according to a first embodiment of the present invention, FIG. 2 is a characteristic diagram showing polarization and efficiency of a holographic diffraction grating, and FIG. 3 is a second embodiment of the present invention. FIG. 4 is a block diagram of a frequency-stabilized light source in an example, FIG. 4 is a block diagram of a frequency-stabilized light source in a conventional embodiment, and FIGS. 1 ... Semiconductor laser device, 2 ... Lens, 43 ... Echelette grating, 4 ... Holographic diffraction grating, 35 ...
Holographic concave diffraction grating.

Claims (4)

[Claims] 1. The lattice constant d is 0.2 <λ / with respect to the used wavelength λ.
A frequency-stabilized light source composed of a holographic diffraction grating having a range of d <1.7 and a semiconductor laser device. 2. A frequency-stabilized light source according to claim 1, wherein a plane diffraction grating is used as the diffraction grating. 3. A frequency stabilized light source according to claim 1, wherein a concave diffraction grating is used as the diffraction grating. 4. A frequency-stabilized light source according to any one of claims (1), (2), and (3), wherein the semiconductor laser device is provided with an antireflection film.