JPH0636454B2 - Semiconductor laser wavelength stabilizer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a semiconductor laser mainly used as a light source for optical communication, optical application meter side, optical information processing, and more specifically, stabilization of laser wavelength and spectral line. The present invention relates to a semiconductor laser wavelength stabilization device that simultaneously realizes a narrow band.

<Prior Art> Semiconductor lasers have advantages such as small size, high efficiency, variable wavelength, low cost, and high speed operation.
Since there are few lasers that oscillate in the wavelength range of about 6 μm, it is expected to be applied to precision measurement and high resolution spectroscopy. For these applications, particularly important characteristics required for semiconductor lasers are high spectral purity and frequency stability, and these two factors greatly affect the sensitivity and resolution of measurement.

Conventionally, as one of the frequency stabilizers for semiconductor lasers, a device using a Fabry-Perot etalon shown in FIG. 10 is known.

In FIG. 10, 1 is a semiconductor laser, which is driven by a current from a constant current source 7. Laser light from the semiconductor laser 1 is collimated by a lens 2, passes through a diaphragm plate 3, an optical isolator 4, and is split into two directions by a beam splitter 5. One of the branched lights is guided to a reference Fabry-Perot etalon (hereinafter referred to as an etalon) 6, where it is attenuated in power according to the wavelength-transmittance characteristic of the etalon 6 and is emitted. The light emitted from the etalon 6 is photoelectrically converted by the photodiode 8 through the diaphragm plate 3a, and its electric signal V _B is input to the divider 10. On the other hand, the other laser beam split by the beam splitter 5 is reflected by the mirror 13 and reflected by the photodiode 9
The electric signal V _{A which} is incident on the optical signal and is photoelectrically converted therein is also input to the divider 10, and the output V _B from the photodiode 8 is divided by the output V _A from the photodiode 9. The output from the divider 10 is input to the inverting input terminal side of the differential amplifier 11, and the difference from the reference input voltage 13 is amplified. The output of the control circuit 12 is controlled by the output of the differential amplifier 11, and the output of the constant current source 7 is controlled based on this output.

With the above configuration, the current from the constant current source 7 is controlled, and the oscillation wavelength of the semiconductor laser 1 is fixed at one point on the wavelength-transmittance characteristic of the etalon 6 for stabilization.

FIG. 11 is a configuration explanatory view showing a conventional example in which a feedback band is widened by using an injection current as a feedback amount to narrow a spectral line width. In the figure, the same elements as those in FIG. 10 are designated by the same reference numerals.

In FIG. 11, reference numeral 30 is a temperature controller for keeping the temperature of the semiconductor laser 1 constant. A semiconductor laser 1 driven by a constant current source 7 passes through a lens 2, an aperture plate 3, an optical isolator 4 and a beam splitter 5 to a beam splitter 2.
Separated into directions.

The light transmitted through the beam splitter 5 is used as output light. On the other hand, the light reflected by the beam splitter 5 enters the etalon 6. The etalon 6 emits light whose power is attenuated according to the wavelength-transmittance characteristic, and this light enters the avalanche photodiode 22 through the lens 2 and the diaphragm plate 3. This avalanche photo
The electric signal photoelectrically converted by the diode 22 is amplified by the high frequency amplifier 24, and the output from the high frequency amplifier 24 is fed back to the semiconductor laser 1 via the capacitor 25.

According to the above configuration, since the AC signal having a high frequency band is fed back through the capacitor 25, the spectral line width can be narrowed.

<Problems to be Solved by the Invention> However, according to the conventional example shown in FIG. 10, since the divider is used, the configuration is complicated, and the feedback band cannot be widened. However, the conventional example shown in FIG. 11 is effective in narrowing the spectral line width, but is not effective in stabilizing the wavelength, and therefore has a problem in practical use. . The present invention has been made in view of the above-mentioned problems of the prior art, and stabilizes the wavelength and narrows the spectral line width by selecting an injection current having a high response speed as the feedback amount and widening the feedback band. It is an object of the present invention to provide a semiconductor laser wavelength stabilizing device that achieves band-passing and stabilizes a feedback loop.

<Means for Solving Problems> A semiconductor laser arranged in a constant temperature bath, a beam splitter for splitting light from the semiconductor laser into two directions, and one light from the beam splitter is converted into an electric signal. A first photoelectric conversion element, a first amplifier that amplifies an electric signal from the first photoelectric conversion element, and a wavelength selection element on which the other light branched by the beam splitter enters.
A second photoelectric conversion element that converts the light emitted from the wavelength selection element into an electric signal, the electric signal from the first amplifier and the electric signal from the second photoelectric conversion element are input, and the difference between them is calculated. The integrator that integrates and the output from this integrator
The amplifier is configured to be fed back to the injection current of the semiconductor laser, and a wide band is used for the integrator and the second amplifier, and the band is limited to a range in which the phase delay of the first amplifier is small. Then, the band of the first amplifier is made narrower than the band of the integrator and the second amplifier, whereby the spectral line width is narrowed and the control system is stabilized.

<Embodiment> FIG. 1 is a circuit configuration diagram (a) and a control system block diagram (b) showing an embodiment of a semiconductor laser wavelength stabilizing device of the present invention. In FIG. 1 (a), LD is a semiconductor laser diode (hereinafter referred to as LD).
Is placed in a thermostatic chamber (not shown) in order to suppress wavelength fluctuations due to temperature fluctuations, and its temperature is controlled to be constant by a temperature controller TC. Then, the wavelength stabilization circuit is driven by a predetermined constant current and operates after receiving a signal from a temperature discriminator (not shown) for discriminating that the temperature inside the constant temperature bath has reached the predetermined temperature. Is configured. The light emitted from the LD is collimated by the lens L into parallel light, and split by the half mirror HM into reflected light used for feedback and transmitted light as practical light. The return light reflected by the half mirror HM is a ½λ plate Z
After passing through the beam splitter PBS, one of the light beams transmitted through the beam splitter PBS is converted into an electric signal by the _first photoelectric conversion element (for example, photodiode) PD ₁ and then the first amplifier A ₁ is inverted. It is input to the input terminal. On the other hand, the light reflected by the beam splitter PBS is a wavelength selection element (eg etalon, diffraction grating, gas cell, etc.)
The light that is incident on E and is attenuated in power according to its wavelength-transmittance characteristic is converted into an electric signal by the _second photoelectric conversion element PD ₂ , and is then fed back together with the output from the _first amplifier A _1. It is input to the inverting input terminal of an integrator B having a resistor R ₀ and a feedback capacitor C. The output of this integrator is connected via a resistor R ₂ to the base of a transistor T _r that constitutes a _second amplifier A ₂ together with the output from Vin via a resistor R ₃ . The collector of the transistor T _r is connected to the LD via the resistor R5, the emitter-side resistor R
It is grounded via ₄ .

In the above configuration, L based on the fed back base voltage
The drive current I flows through D.

FIG. 1 (b) is a block diagram of the control system of the above circuit configuration diagram, and the respective symbols are as follows. , Λ ( _I ); Current-wavelength characteristic of LD P; Current-optical power conversion coefficient of LD D ₁ , D ₂ ; Distribution ratio of outgoing light power distributed by 1/2 λ plate and beam splitter PBS 0 <D ₁ + D ₂ ≦ 1 K ₀ ; sensitivity of photoelectric conversion elements PD ₁ and PD ₂ G ₁ ; gain of _first amplifier A _{1 at} the subsequent stage of photoelectric conversion element PD ₁ ; throughput of wavelength selection element E 0 <T <1 g ( λ); Transmittance characteristic of the wavelength selection element E (standardized so that the maximum transmittance is 1) G ₂ ; Second amplifier A ₂ (transistor circuit)
Voltage-current conversion coefficient of the above, using the above symbols, the relationship between the wavelength (λ) of the laser light and the current I injected into the semiconductor laser is as follows.

λ = Λ ( _I ) (1) I = G ₂ · Vin + G ₂ {-(1 + sCR ₀ ) / (sC)} ・ (G ₁ K ₀ D ₁ PI −K ₀ g (λ) TD ₂ PI) ... (2) where s is the complex number (ωj) (1), if I is eliminated from (2), λ = Λ (G ₂ · Vin / [1 + {(1 + sCR ₀ ) / (sC)} G ₂ K ₀ P (G ₁ D ₁ −TD ₂ g (λ))]) (3) Here, since the oscillation wavelength λ of the LD has the characteristics shown by the alternate long and short dash line in FIG. 2 with respect to the injection current I, The vicinity of the current of interest can be represented by a straight line (a) as shown by a solid line.

This solid line is represented by the following equation.

Λ ( _I = ΛI + λ ₀ where λ ₀ is the value at the point where the assumed straight line and the vertical axis in Fig. 2 intersect (3) [1 + {(1 + sCR ₀ ) / (sC)} G ₂ K ₀ P ( G ₁ D ₁ −TD ₂ g (λ))] (λ−λ ₀ ) = Λ · G ₂ · Vin ...
(4) as s = 0 in the state where the wavelength is controlled to a constant value _{G 2 K 0 P (G 1} D 1 -TD 2 g (λ)) · (λ-λ 0) = 0 ... (5) ( To satisfy the expression (5), G ₂ ≠ 0, K ₀ ≠ 0, and P ≠ 0. Therefore, when λ = λ ₀ or TD ₂ g (λ) = G ₁ D ₁ λ = λ ₀ , I = 0 In other words, since it is in a state of not emitting light, it may be considered that λ ≠ λ ₀ . Therefore, g (λ) = (G ₁ D ₁ ) / (TD ₂ ) ... (6) Here, the output of the gain G ₁ of the first amplifier A ₁ shown in FIG. ₁ and the second photoelectric conversion element PD Ier the difference output from the output of ₂
and r, and the λ _f the wavelength, thereby satisfying the expression (6), g
In the region where (λ) monotonically increases, the relationship between the output of the first amplifier A _{1 and} the output of the second photoelectric conversion element PD ₂ is as shown in FIG.

As can be seen from FIG. 3, when λ <λ _f , I err> 0, then I decreases and λ is small, and when λ> λ _f , I err <0, I increases and λ is large. Becomes

Therefore, since positive feedback occurs, the wavelength cannot be stabilized at λ _f .

Further, in the region where g (λ) monotonously decreases, the relationship between the output of the first amplifier and the output of the second photoelectric conversion element PD ₂ is as shown in FIG.

As can be seen from FIG. 4, when λ <λ _f , Ieff <0, and then I increases and λ is large, and when λ> λ _f , Ieff> 0, and I decreases and λ is small. Becomes

Therefore, it becomes a negative feedback, and the wavelength can be stabilized at λ _f . That is, the wavelength can be stabilized at a specific position of g (λ).

FIG. 6 is a combination of FIG. 3 and FIG. 4, with the vertical axis representing 1 / TD.
_It is scaled to ₂ . In Fig. 6, λ
_The neighborhood of _f is approximated by a straight line, g (λ) = α (λ−λe) (α <0) where α is the slope of the straight line λ _e is the value at the point where the approximated straight line intersects the horizontal axis λ _f = ( G ₁ D ₁ ) / (TD ₂ α) + λ _e .

Actually, since g (λ) has the periodicity shown in FIG. 5, if the initial value of λ is set within the range of the target one period by Vin, only one wavelength will be stabilized. .

Here, when the formula (6) is modified, G ₁ D ₁ / (Tg (λ) D ₂ ) = 1, which is V _B / V _A = of the conventional example shown in FIG.
This is equivalent to applying feedback so that it is constant. Therefore, it is possible to perform the same control as when the divider is used without using the divider.

Further, since the high frequency component of the power variation of the etalon transmitted light, which uses a wide band, is fed back to the integrator B and the second amplifier A ₂ , the spectral line width can be narrowed.

According to the above configuration, the wavelength is the characteristic of the etalon (α, λ _{e in} Equation 7) and the gain before differential (G ₁ in Equation 7,
It is determined only by D ₁ , T, D ₂ ) and does not depend on the coefficient Λ connecting the injection current and the wavelength, the optical power, or the input value. (However, since the characteristic g (λ) of the etalon has a periodicity, it is necessary to set the input value with Vin in order to select a specific period.) First photoelectric conversion element PD1 to first amplifier The signal input to the integrator B via A ₁ is the second photoelectric conversion element PD 2
There is a phase delay that occurs in the _first amplifier A ₁ (G ₁ ) compared to the signal input directly to the integrator B from. When this phase delay amount becomes large, the difference between the signal having a small phase delay and the signal having a large phase delay is fed back to the fluctuation of the light intensity of the LD, and the feedback loop becomes unstable. In order to make this feedback loop stable, it is necessary to reduce the amount of phase delay of the first amplifier. Therefore, the band is limited to the range where the phase delay of the first amplifier A ₁ is small.

FIG. 7 is a circuit configuration diagram (a) and a control system block diagram (b) showing another embodiment. 7 (a) and 7 (b), the same elements as those in FIG. 1 are designated by the same reference numerals and duplicate description is omitted, but in this example, the first and second photoelectric conversion elements PD ₁ and The orientation of PD ₂ is reversed. Therefore, in the block diagram shown in FIG. 2B, the sign of the output of the first amplifier A ₁ (G ₁ ) and the sign of the output from the _second photoelectric conversion element PD ₂ are opposite to those in FIG. 1B. become. Therefore, the rate of change α of the transmittance of the etalon depending on the wavelength is α>
When 0, wavelength stabilization is possible.

FIG. 8 is a circuit configuration diagram (a) and a control system block diagram (b) showing still another embodiment. FIG. 8 (a),
In (b), the same elements as those in FIG. 1 are designated by the same reference numerals and the duplicate description is omitted, but in this example, the diffraction grating U is used instead of the etalon. Since the diffraction grating has different directions in which the optical powers are strengthened according to the wavelength, if the incident light, the diffraction grating, and the observation point are fixed, the characteristics shown in FIG. 9 can be obtained for the wavelength of the incident light. Therefore, it is possible to stabilize the wavelength and narrow the spectral line width by the same operation as that shown in FIG.

<Effects of the Invention> According to the present invention as described above in detail with reference to the embodiments, the feedback from DC to high frequency is performed at the same time. Therefore, the wavelength stabilization and the narrowing of the spectral line width can be easily performed. Can be realized at the same time. Further, since the temperature controller and the semiconductor laser stabilizing circuit are separately operated so that the device is operated after reaching a predetermined temperature, even if the power is turned off and then turned on again, the same wavelength is obtained. Moreover, the oscillation wavelength of the LD can be stabilized.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram (a) showing an embodiment of a semiconductor laser wavelength stabilizing device of the present invention, and a control system block diagram (b), and FIG. 2 is a diagram showing current wavelength characteristics of a semiconductor laser diode, 3 and 4 are diagrams showing the relationship between the output of the first amplifier A _{1 and} the output of the second photoelectric conversion element PD ₂ in the region where g (λ) monotonically increases and monotonically decreases, and FIG. 5 shows FIG. 6 is a relationship diagram showing the periodicity of g (λ) with respect to λ, FIG. 6 is a relationship diagram showing a state in which the vicinity of λ _f is approximated by a straight line, FIG. 7,
FIG. 8 is a diagram showing another embodiment, FIG. 9 is a diagram showing a relationship between wavelength (λ) and light intensity when a diffraction grating is used,
FIG. 11 and FIG. 11 are configuration explanatory views showing a conventional example. A ₁ ... First amplifier, A ₂ ... Second amplifier, B ...
Integrator, E ... Wavelength selection element, L ... Lens, LD ...
Semiconductor laser diode, PD ₁ , PD ₂ ... Photoelectric conversion element, PBS ... Beam splitter, R _{1 to} R ₅ ... Resistor, Z ... 1 / 2λ plate.

Claims (3)

[Claims] 1. A semiconductor laser arranged in a constant temperature bath, a beam splitter for splitting light from the semiconductor laser into two directions, and a first photoelectric converter for converting one light from the beam splitter into an electric signal. A conversion element, a first amplifier for amplifying an electric signal from the first photoelectric conversion element, a wavelength selection element on which the other light branched by the beam splitter is incident, and light emitted from the wavelength selection element A second photoelectric conversion element for converting into an electric signal, an integrator for inputting the electric signal from the first amplifier and the signal from the second photoelectric conversion element, and integrating the difference, and this integrator Output from the semiconductor laser is fed back to the injection current of the semiconductor laser through a second amplifier, and a wide band is used for the integrator and the second amplifier, and the phase delay of the first amplifier is In a small range By limiting the band and making the band of the first amplifier narrower than the band of the integrator and the second amplifier, the spectral line width is narrowed and the control system is stabilized. A semiconductor laser wavelength stabilizer. 2. A temperature controller and a semiconductor laser wavelength stabilizing circuit are operated separately, and first, the temperature controller sets the temperature in the thermostatic chamber to an initial set temperature, and the temperature discriminating circuit indicates that the temperature has reached a predetermined temperature. The semiconductor laser wavelength stabilizing device according to claim 1, characterized in that the laser wavelength stabilizing circuit is operated after confirmation. 3. A first photoelectric conversion element post-stage in which the ratio of the power of light incident on the wavelength selection element and the power of light used for feedback without passing through the wavelength selection element or the light input without passing through the wavelength selection element 2. The semiconductor laser wavelength stabilizing device according to claim 1, wherein the wavelength to be stabilized can be varied by changing the gain (G ₁ ) of the first amplifier.