JPS625677A - Frequency stabilized semiconductor laser device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION FIELD OF INDUSTRIAL APPLICATION This invention relates to frequency stabilized semiconductor laser devices used in optical communication and optical measurement.

BACKGROUND OF THE INVENTION In recent years, semiconductor lasers have begun to be used as signal sources and light sources in the fields of optical communication and optical measurement, taking advantage of their good coherence. However, the oscillation frequency of a semiconductor laser is easily influenced by variations in the oscillation frequency, temperature, and returned light, and thus tends to fluctuate. Therefore, semiconductor laser (
By placing a mirror or a diffraction grating outside the LD (hereinafter referred to as LD) and returning the output light from one end face of the LD to its light emitting point, a composite resonator is formed, and single-wavelength oscillation and oscillation frequency selection are performed. . Past embodiments will be described with reference to FIG. Is Figure 3(a) a mirror? L with the composite resonator laser used
The output light of D23 is converted into parallel light by all lenses 22 and is incident on the mirror.
The reflected light returns to the light emitting point of the LD 23 through the original optical path. As a result, the Q value of the resonator increases and the resonator oscillates at a single wavelength, and by changing the resonator length Li, the oscillation conditions can be changed and the frequency can be changed. In FIG. 3(b), a diffraction grating is used instead of the mirror, and the incident light from the LD 26 is diffracted by the diffraction grating 24 so as to satisfy a grating equation for each wavelength. IJ) In the case of a low configuration, where the incident angle is 2, the diffraction angle is 20, the lattice constant is d, and the wavelength is λ, light with a wavelength of λ is diffracted in the θ direction that satisfies the following conditions: 2 d sin θ=mλ−(1).

When the diffraction grating is tilted to an angle θ that satisfies equation (1), the oscillation wavelength of the LD 26 becomes λ. Also, by changing the angle θ, the LD26
It is possible to change the oscillation wavelength based on equation (1).

Problems to be Solved by the Invention In the above-mentioned LD in which the external resonator is biased toward gold, the refractive index and energy bandgap of the LD change due to temperature changes around the LD and heat generated while the LD itself is driven. Even when oscillating with a single wavelength, that wavelength may cause mode hops or fluctuations due to temperature and other influences.As a result, in addition to frequency instability, the output intensity also changes, making communication difficult. In systems, it causes noise, and in measurements, it becomes a major cause of measurement errors.

In view of the above-mentioned problems, the present invention provides a frequency-stabilized semiconductor laser that stabilizes the oscillation frequency of the LD and keeps its output power constant.Means for Solving the ProblemsSolving the above-mentioned problems In order to achieve this, the frequency-stabilized semiconductor laser of the present invention includes a semiconductor laser element, a lens, a diffraction grating two-position detection element, and a photodetection element, and returns the first-order diffracted light generated by the diffraction grating to the semiconductor laser to generate a single wavelength. Along with the oscillation, a photodetector detects the change in the intensity of the 0th-order diffracted light, adjusts the drive current of the semiconductor laser, and focuses the 2nd-order diffracted light on the focal plane with a condensing lens to determine the position of the f-quadrant. The temperature of the semiconductor laser is controlled so that the change is detected by a position detection element and the image is formed at a fixed position. Embodiment 0 A frequency-stabilized semiconductor laser on the one-shot side of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a frequency-stabilized semiconductor laser according to a first embodiment of the present invention. In the first factor, 5 is a semiconductor laser, and the output light from one end face of this semiconductor laser (hereinafter referred to as LD) 5 is made into parallel light by a collimator lens 6 and is incident on the diffraction grating 1 . Now, if the oscillation frequency of LD5 is λ0, the angle α between the incident light and the normal to the diffraction grating is
and the diffraction angle β is d (Sin α+ Sinβ) = mλo -----
-・(2) is the relationship. d is a lattice constant 1m is an integer.

IJ for m21) When the diffraction grating is tilted to the low configuration, α = β = θ, 2a sin θ = λo -=---- (3), and the first-order diffracted light 16 is returned to the light emitting point of LD5. Therefore, the oscillation frequency of the LDs is limited to around λ0.

In the combination of the first-order diffracted light 16 from the diffraction grating and the LDs, the spot on the light emitting surface of the returned light has a width approximately equal to the oscillation wavelength at the diffraction limit of the collimator lens, and is generally 0.8~ Since the blur is about 1.6a, the oscillation wavelength λ0
There is also.

It can vary within the range of Δλ. Therefore, in equation (1), m
= 2f Second-order diffracted light is used to detect the wavelength fluctuation. In equation (1), the oscillation wavelength λ0 of LD5 is λ
If it changes to 0 + Δλ, then 2d CO3θΔθ = 2・Δλ ... (4) Δ
θ=Δλ/dcosθ (5) A diffraction angle change of Δθ occurs. Therefore, when this second-order diffracted light 18 is imaged by the condensing lens 7, on the focal plane of the lens 7, ΔX= f・Δθ=f・Δλ/ co sθ...
...(6) causes the position change. However, f is the focal length of the lens 7. If a position detection element 2 such as a PSD or a two-split photodiode is placed on the focal plane of the lens 7, the position caused by the wavelength change of the diffracted light can be detected, and the error detection section 3 Using the output error signal, L
The temperature of D5 is electrically controlled using a Peltier element or the like, and the temperature of the LDs is controlled so as to return to the original position. Therefore, by applying the position error signal to the temperature control system, the oscillation frequency of the LDs can be kept constant.
Further, the resolution δ of the diffraction grating is expressed as δ=Δλ/λ=mW (7). m is the order, and N is the total number of lattices.

By using the second-order diffracted light 18, the wavelength change Δλ can be detected with higher sensitivity than the first-order diffracted light. In general, as m increases, the intensity of the diffracted light decreases, so it is preferable to use it with aperture = 2. For higher-order diffracted light of Om = 2 or more, the resolution δ increases, but due to the decrease in intensity, the S / N decreases, and the overall effect of noise on the element becomes easier.0 The 0th-order diffracted light generated by the diffraction grating 1 is generated by specular reflection of the diffraction grating, and therefore has no wavelength dependence. This O
A photodetector detects changes in the light intensity of the secondary light 17' and feeds back to the drive circuit 9 to keep the output light intensity constant.

FIG. 2 is a structural diagram of the lens and diffraction grating shown in FIG. 1, which are all integrated. Lenses 6, 7 in FIG. 8
Instead, a glass block is used, and the three surfaces perpendicular to the 0th, 1st, and 2nd order diffraction directions determined by the angle θ that satisfies equation (1) for the desired oscillation frequency λ of the semiconductor laser 6 are used as lenses. Process into shapes. Since this optical system allows a very narrow angle of view, the lens surface can also be an aspherical paraboloid, making it possible to create an optical system free of aberrations. The diffraction grating portion 15 may be formed by directly forming the entire grating on the glass material, or by gluing a commercially available planar grating onto the glass block 14 using silicone oil or ultraviolet curing resin. By making the entire system into a glass block in this way, it is possible to realize a system that is compact and relatively stable against mechanical vibrations.

Effects of the Invention As described above, the present invention includes three lenses and a diffraction grating outside a semiconductor laser element, a position detection element that detects a change in the position of the second-order diffracted light on the lens focal plane, and a position detection element that detects the change in the intensity of the ○th-order diffracted light. A temperature control circuit includes a photodetector for detecting the semiconductor laser element, and uses a signal from the position detection element to control the operating temperature of the semiconductor laser element; By providing a drive circuit that controls the emission current of the laser element, it is possible to provide a semiconductor laser element with stable oscillation frequency and emission power.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a frequency-stabilized semiconductor laser according to a first embodiment of the present invention, FIG. 2 is a block diagram of an optical system of a frequency-stabilized semiconductor laser element according to a second embodiment of the present invention, and FIG. 1 is a configuration diagram of a conventional semiconductor laser device. 1.15... Diffraction grating, 6... Semiconductor laser element, 2... Position detection element, 11...
...Photodetector, 9...Drive circuit, 4...
...Temperature control section, 6. 7.8...Lens,
14...Glass block.

Claims (2)

[Claims] (1) An optical system including a semiconductor laser element, a collimator, two lenses, and a diffraction grating, a position detection element, and a photodetector, the output light from one end surface of the semiconductor laser element is transmitted by the collimator lens. Parallel light,
The first-order diffracted light generated by the diffraction grating is made to enter the diffraction grating, and the first-order diffracted light is returned to the light emitting point of the semiconductor laser element, and the second-order diffracted light is imaged on the position detection element by the condensing lens. a temperature control section that controls the temperature of the semiconductor laser element according to an output signal from the semiconductor laser element; A frequency-stabilized semiconductor laser device characterized by having a drive circuit that controls current. (2) A frequency-stabilized semiconductor laser device according to claim (1), in which the diffraction grating and three lenses are integrated with a glass block.