JPH0663866B2 - Wavelength detector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a device such as a semiconductor laser that stabilizes an oscillation wavelength oscillated by an ambient temperature or an injection current.

(Prior art and its problems) Semiconductor lasers have many features such as small size, low power consumption, relatively high optical output, and direct high-speed modulation. It has a wide range of applications such as optical information processing. As is well known, the semiconductor laser has a characteristic that the refractive index of the crystal changes due to the ambient temperature or heat generated by the injection current, the optical resonator length changes, and the oscillation wavelength easily changes. Generally, the rate of change of wavelength is about 3 to 4 angstroms per degree of temperature.
In a normal optical communication device that uses semiconductor laser light as energy, an optical disc device, or the like, the above-mentioned temperature fluctuation of the wavelength does not have a great influence on system performance. However, when an optical phenomenon whose characteristics depend on the wavelength, such as the diffraction effect of light by a grating or the interference effect of light waves, is used, the temperature dependence of the oscillation wavelength of the above semiconductor laser is too large. Hard to use. For example, one example of a device that uses the diffraction effect of light by sound waves, the resolution of the spectrum analysis of an acousto-optic spectrum analyzer that detects the frequency spectrum of the high-frequency signal applied to the ultrasonic cell by the diffraction position of the light diffracted by the cell is It is difficult to use a semiconductor laser as a light source because it is affected by the stability of the wavelength. Further, an optical sensor that uses light interference, for example, in a circularly wound optical fiber, the light waves that pass clockwise and counterclockwise with respect to this circular loop are caused to interfere with each other, and the plane formed by the fiber loop In the optical fiber gyro device that detects the phase difference between the two light waves as the interfering light intensity change, it is difficult to set the optical path difference between the left and right lights exactly the same. The fluctuation of the wavelength of the light source causes a change in the intensity of the interference light through the optical path difference. That is, a drift is added to the detection signal, resulting in a decrease in measurement accuracy.

A well-known technique for detecting a wavelength is a spectroscope technique. It radiates the light to be measured on a dense sawtooth reflection grating with spatial frequency, reflects and diffracts it, and rotates the reflection grating so that it passes through a slit located in a specified direction, and the wavelength can be known from the rotation angle. is there.

One of the advantages of using a semiconductor laser as the light source of an optical device is
It is small, and the entire device is small.
As seen in an optical pickup of an optical disk device, a peripheral light circuit for converting a semiconductor laser radiation into a parallel beam and splitting the polarized light is only about the size of a cigarette box. It is difficult to use the above-mentioned spectroscopic technique to detect the wavelength fluctuation of the semiconductor laser in such a small device.In the present method of detecting the wavelength change from the diffraction angle of the reflection grating, the wavelength discrimination resolution is This is because the optical path length from the reflection grating to the slit must be long in order to increase the height. For this reason, the device size cannot be avoided.

(Object of the Invention) An object of the present invention is to eliminate the above-mentioned conventional drawbacks and to provide a small-sized wavelength detection device excellent in mass productivity.

(Structure of the Invention) According to the present invention, a part of a light beam obtained by converting a laser beam having a central wavelength λ into a parallel light beam is separated from an optical path, and a sinusoidal diffraction grating having a grating pitch α is approximately sin θ _i + 1 = λ / α. Incident at an incident angle θ _i and transmitted and diffracted by the diffraction grating,
By measuring the difference in the intensity of the reflected diffracted light, a wavelength detection device for detecting the change in the wavelength λ of the laser light can be obtained.

(Principle of Operation of the Present Invention) Next, the present invention will be described in detail with reference to the drawings. Second
The figure is a cross-sectional view showing the relationship between incidence and diffraction of light on a diffraction grating in order to explain the principle of the present invention.

In the figure, the lattice is enlarged from the actual size to show that 11 is a lattice plane. In FIG. 2, incident light 12 is incident from the substrate surface 13 of the grating at the incident angle θ ₁ shown in the figure. Incident light is refracted on the substrate surface, and the refraction angle θ ₂ is given by the following equation, where n is the refractive index of the substrate.

sin θ ₁ = ns in θ ₂ (1) The refracted light 14 is diffracted on the grating surface 11 according to the following equation, and becomes diffracted light 15 shown by a dotted line and goes out into the air.

nsinθ ₂ + sinθ ₃ = λ / α (2) where λ is the wavelength of light and α is the pitch of the grating.
Substituting equation (1) into equation (2) yields the following equation.

sin θ ₁ + sin θ ₃ = λ / α (3) Consider the case where λ / α is larger than 1 in the equation (3). When the incident angle θ ₁ is decreased from 90 ° to 0 °, the diffraction angle θ ₃ increases, and when the incident angle θ ₁ is sin θ ₁ = λ / α-1 (4) , Θ ₃ becomes 90 °, and the diffracted light does not come out into the air. At this time, the following equation is established in the substrate.

nsinθ ₂ + nsinθ ₄ = λ / α (5) Therefore, nsinθ ₄ = 1 and reflected diffracted light 16 is generated. However, since sin θ ₄ = 1 / n, the diffracted light 16 is totally reflected by the substrate surface 13 and becomes reflected light 17. Reflected light 17
Is a mirror symmetry with the diffracted light 16 and is reflected by the grating surface 11
Yields 18. The reflected diffracted light 18 is refracted at the substrate surface 13,
It becomes 19 and goes out into the air. As explained above, θ in Eq. (4)
_When sin θ ₁ +1> λ / α with ₁ as the boundary, the diffracted light 19 is not generated and the diffracted light 15 is generated from the grating surface side.

When sin θ ₁ +1 <λ / α, diffracted light 19 is generated, and diffracted light 15 on the grating surface side is not generated. Therefore, diffracted light 19 and diffracted light 15
By differentially detecting and,
It is possible to detect the deviation from the value that satisfies / α = sin θ ₁ +1. Further, as a different detection method, the incident angle θ ₁ of the light beam on the grating is set in advance so that diffracted light 15 is generated, that is, sin θ ₁ +1> λ / α.

The output angle θ ₃ of the diffracted light 15 is calculated from the formula (3) as follows: θ ₃ = sin ⁻¹ (λ / α-sin θ ₁ ) ... (6) Therefore, the output angle θ ₃ of the diffracted light 15 is changed by the change of the wavelength λ. Therefore, the two-divided photodetector can detect this angle change, that is, the wavelength change.

The above is the principle of the present invention.

The same effect can be obtained by reversing the front and back of the diffraction grating used in the present invention.

FIG. 3 is a sectional view showing the principle of the present invention when the front and back of the diffraction grating of FIG. 2 are reversed. Also in this figure, the lattice is enlarged from the actual size to show that 21 is a lattice plane. In FIG. 3, incident light 20 is incident from the grating surface 21 of the grating at θ ₅ shown in the drawing.

The incident light is diffracted on the lattice plane, and the diffraction angle θ ₆ has the following relationship, where n is the refractive index of the substrate.

sin θ ₅ + nsin θ ₆ = λ / α (7) The diffracted light 22 is refracted on the substrate surface 23 according to the following equation and becomes refracted light 24 shown by a dotted line and goes out into the air.

nsin θ ₆ = sin θ ₇ (8) Substituting equation (7) into equation (6) yields the following equation.

sin θ ₅ + sin θ ₇ = λ / α (9) Again, consider the case where λ / α is larger than 1 in the equation (8).

When the incident angle θ ₅ is decreased from 90 ° to 0 °, the refraction angle θ ₇ increases, and when the incident angle θ ₅ is sin θ ₅ = λ / α-1 (10), θ ₇ becomes 90 °, refracted light does not come out in the air, and is totally reflected by the substrate surface 23. Totally reflected light 25 is a lattice plane
It is diffracted again at 21 and becomes diffracted light 26 and goes out into the air. As explained above, with θ ₅ of the equation (9) as a boundary, sin θ ₅ +1>
When λ / α, diffracted light 26 does not occur, and refracted light 24
Cause When sin θ ₅ +1 <λ / α, diffracted light 26 is generated and refracted light 24 on the substrate surface side is not generated. Therefore, similarly to the above, the diffracted light 26 and the refracted light 24 are differentially detected or the refracted light 24 is detected.
The wavelength change of the light source can be detected by detecting the change in the angle.

(Embodiment) FIG. 1 is a block diagram showing an embodiment in which the device of the present invention is applied to a wavelength stabilizing device. The emitted light of the semiconductor laser 1 which is the light source is collimated by the collimating lens 2. A part of the light flux is separated by the beam splitter 3 inserted in the optical path of the parallel beam and is incident on the diffraction grating 4. The grating pitch α of the diffraction grating 4, the center wavelength λ ₀ of the semiconductor laser 1, and the incident angle θ _i to the diffraction grating are set so as to substantially satisfy sin θ _i + 1 = λ ₀ / α. The oscillation wavelength of the semiconductor laser 1 changes due to fluctuations in the ambient temperature. When the ambient temperature decreases and the oscillation wavelength shifts to a short wavelength, the light 9 emitted from the surface of the diffraction grating 4 opposite to the light incident side appears as described above. When the wavelength is shifted to the long wavelength, the light 10 emitted from the surface on the light incident side appears. Each of them is received by the photodetectors 5 and 6 and differentially amplified by the amplifier 7, and an injection current is given to the Peltier device 8 installed in the semiconductor laser 1. This feedback system stabilizes the oscillation wavelength to λ ₀ .

As a numerical example of the diffraction grating, when used at λ = 780 nm, assuming that the incident angle is approximately 45 °, the grating pitch is 0.4569 from equation (4).
μm. Such a pitch grating can be easily manufactured by two-beam interference. To record directly to photoresist,
Incident angle 28.897 using He-Cd laser with wavelength 441.6 μm
It can be obtained by interfering two light beams at an equal incident angle of °.

As is well known, the wavelength of the semiconductor laser changes not only with the temperature but also with the injection current. Wavelength stabilization can be realized even if the injected current is fed back to the differential output of the two-divided detector.

(Effect of the Invention) The diffraction grating used in the present invention is manufactured as a surface relief grating on a photoresist as described above, and then a metal mold is manufactured by a nickel electroforming method. Can be mass-produced. It can also be manufactured by etching the glass surface by photolithography.

As described above, according to the present invention, it is possible to obtain a wavelength detecting device which is small, inexpensive, and excellent in mass productivity.

[Brief description of drawings]

FIG. 1 is a block diagram showing the first embodiment of the present invention, FIG.
FIG. 3 is a sectional view of a diffraction grating showing the principle of the present invention. In the figure, 1 ... semiconductor laser, 2 ... collimating lens, 3 ... beam splitter, 4 ... diffraction grating, 5,6 ... photodetector, 7 ... amplifier, 8 ... Peltier element, 11 , 21 …… Lattice plane, 13,23 …… Substrate plane.

Claims (1)

[Claims] 1. A laser beam having a central wavelength λ is incident on a transmission diffraction grating having a grating pitch α at an incident angle θ _i of sin θ _i + 1 = λ / α, and the transmitted diffraction light and the reflection diffraction of the diffraction grating are incident. A wavelength detecting device, which detects a change in wavelength of the laser light by measuring a difference in light intensity.