JPS6343905B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser device in which the difference frequency between the oscillation frequencies of two semiconductor lasers is highly stable.

Conventional devices of this type consist of two laser oscillation devices, the oscillator length of both or one of the lasers is controlled by PZT, and the difference frequency between the oscillation frequencies of the two lasers is set to a reference value. When the difference frequency fluctuates from the reference frequency, the voltage applied to the PZT is changed according to the fluctuation, and the oscillation frequency of one or both lasers is changed, thereby maintaining the difference frequency again at a value close to the reference frequency. In this case, since the resonator length is mechanically varied, it is susceptible to mechanical vibrations and has the drawbacks of slow control response.

In order to eliminate these drawbacks, the present invention maintains each semiconductor laser device stably, and converts the output light of one laser using a frequency converter to stabilize the difference frequency. When a frequency fluctuation occurs, the conversion frequency of the frequency converter is changed.In this case, reflecting the characteristics of the frequency converter, it is less susceptible to mechanical vibrations, and it is also easier to control. It has the characteristics of a quick response and is less susceptible to thermal fluctuations.

FIG. 1 shows an embodiment of the present invention. 1 is a semiconductor laser, 2 is a temperature control device, 3 is a temperature control circuit, 4 is an interference filter as a frequency discriminator, 5 is a photodetection element, 6 is a control signal generation circuit, 7 is a variable optical frequency converter, and 8 is a In its driving circuit, 9 is a half mirror, 10 is a photodetecting element that generates a difference frequency, and 11 is a control signal generating circuit.
The output light L ₁ , L ₂ of the semiconductor laser itself is 1 to 6
The device configuration keeps the frequency stable.

First, this point will be explained.

FIG. 2 is an extracted structure of one of the laser devices shown in FIG. 1 in order to explain how to stabilize each of the two laser beams L ₁ and L ₂ in terms of frequency.

One of the output lights L ₁₁ and L ₁₂ of the semiconductor laser 11, L ₁₂ , is passed through an interference filter 14, which is a type of frequency discriminator, and the intensity of the output light L ₁₃ is detected by a photodetector element 15. The interference filter 14 generally has transmittance characteristics as shown in FIG. Then, the optical path length of the resonator is changed by thermal expansion, electro-optic effect,
Vibration is caused to change at a predetermined period using a piezoelectric effect or a mechanical method. This is equivalent to changing the frequency of the input light _L12 vibrationally as shown in Figure 3, and therefore the intensity of the output light _L13 is
It changes vibrationally as shown. This output light L ₁₃
Measuring the change in intensity means measuring the differential coefficient of the transmission characteristic of the interference filter 14, and thereby the frequency of the output light of the semiconductor laser 11 becomes
By generating an appropriate feedback signal S ₁ when the frequency deviates from the peak frequency ω ₀ of the transmission characteristic and applying it to the temperature control circuit 13 of the semiconductor laser 11, the output optical frequency of the semiconductor laser 11 is set to ω ₀ again. can be restored to. As a result, the frequencies of the output lights L ₁₂ and L ₁₃ are held almost constant at the value of ω ₀ .

In the above, an example has been explained in which the resonator length of the interference filter 14 is changed oscillally.
Semiconductor laser 11 with the cavity length kept constant
Exactly the same effect can be obtained by inserting an optical frequency converter between the interference filter 14 and the interference filter 14, thereby vibrationally changing the frequency of the light incident on the interference filter 14.

Furthermore, if the vertical axis in Figure 3 is replaced with the absorption coefficient of an atom or molecule, the curve of the absorption spectrum is similar to the curve of the characteristics of the interference filter in Figure 3, so a detailed explanation will be omitted. A second container containing a gas of atoms or molecules of
It can be used in place of the interference filter 14 in the embodiment shown. In particular, cesium atoms are suitable for semiconductor lasers with a diameter around 0.85 μm. That is, in this way, an absorption type frequency discriminator can be realized.

According to the above explanation, the output light of the laser beam in FIG.
It has become clear that the frequencies of each of L ₁ and L ₂ are maintained stably, but the feature of the device of the present invention shown in FIG. The goal is to keep it stable.

That is, when the output light _L1 of the semiconductor laser and the output light _L3 of the optical variable frequency converter are combined by the half mirror 9 and received by the photodetector element 10, the output signal includes a beat signal of the difference frequency. . Therefore, the beat signal frequency △ω and the reference signal frequency △ω ₀ of the reference signal R are compared in the control signal generation circuit 110, and if a difference occurs, the drive circuit 8 of the variable frequency converter is controlled to suppress the difference. A signal S′ is provided. That is, when the difference frequency between the output lights _L1 and _L2 fluctuates from a predetermined value, the variable frequency converter 7 is operated by the control signal S' to change the frequency of the output light _L3 , and the output The difference frequency between lights L ₁ and L ₃ is always kept at a constant value. Therefore, according to this embodiment, two light beams whose difference frequencies are highly stable can be generated.

As an application example of the device of the present invention, it is advantageous to use it as a light source on the transmitting side in an optical heterodyne communication system in which both signal light and reference light are sent from the transmitting side.

As explained above, according to the present invention, it is possible to realize a semiconductor laser device using a semiconductor laser as a light source with a stable difference frequency, and furthermore, the device is small and is not affected by ambient temperature fluctuations and mechanical vibrations. It has the advantage of being a device with a small amount of energy.

[Brief explanation of the drawing]

Figure 1 shows one embodiment of the device of the present invention, and Figure 2 shows the first embodiment of the device.
FIG. 3 is a configuration diagram illustrating a frequency stabilizing means of one of the laser devices in the figure. FIG. 3 is a diagram explaining the principle of frequency stabilization, and is a diagram showing the relationship between interference filter transmission characteristics and input light L ₁₂ and output light L ₁₃ . DESCRIPTION OF SYMBOLS 1, 11... Semiconductor laser, 2, 12... Temperature control device, 3, 13... Temperature control circuit, 4, 14... Interference filter, 5, 15... Photo detection element, 6, 16... Control signal generation circuit, 7... Light Variable frequency converter, 8...
Drive circuit, 9...half mirror, 10...photodetection element,
110...Control signal generation circuit.

Claims (1)

[Claims] 1. A semiconductor laser, a control device that controls the oscillation frequency of the semiconductor laser by controlling its temperature or injection current, and an absorption or transmission type frequency discriminator that receives a portion of the output light of the semiconductor laser. and a photodetection element that receives the output light of the frequency discriminator and detects the intensity of the output light.
The frequency discriminator is supplied with the output of the photodetecting element and is generated when the frequency of the output light of the semiconductor laser changes in accordance with the value of the differential coefficient of the characteristic curve of the frequency discriminator. and a control signal generation circuit that detects intensity fluctuations in the output light of the output light, and generates a feedback signal that suppresses the frequency fluctuations in response to the intensity fluctuations, and generates a feedback signal that suppresses the frequency fluctuations according to the temperature or injection current. Two semiconductor laser devices configured to supply to a control device, a variable optical frequency converter that receives the output light of one of the semiconductor laser devices, and a variable optical frequency converter that receives the output light of the variable frequency converter and the output light of the other semiconductor laser device. A photodetecting element capable of detecting an optical beat signal with the output light, and the optical beat signal and a reference signal of a predetermined constant frequency are supplied, and when the frequency of the optical beat signal fluctuates from the frequency of the reference signal. A semiconductor laser device comprising: a control circuit that operates the variable optical frequency converter to suppress the fluctuation by feeding back a control signal to the variable optical frequency converter according to the fluctuation.