JPH0964449A - Magnetic field-controlled high stable light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain amplitude-stabilized output light from a magnetic field-controlled high stabilized light source with less noise by controlling a magnetic field generated from a magnetic field generator so that the detection signal corresponding to the output light can become a prescribed value. SOLUTION: The output laser beam 6 of a laser 1 travels as laser beams 8, 10, and 12 by branching off an output laser beam 7 and wavelength detecting laser beams 9 and 11 whenever the laser beam 6 passes through semitransparent mirrors 20, 22, and 24. A photodetector 26 detects the intensity of the laser beam and, when a switch 52 is closed, outputs the intensity to a power control source control circuit 5 as a light intensity output. The circuit 50 integrates and amplifies the differences between the light intensity output of the photodetector 26 and modulated signals inputted to a modulated signals inputting terminal 50 and outputs the amplified result to the control terminal M of a power source 3 as a control input. The power source 3 supplies an element current to the element 1 in accordance with the control input. When an error signal represents the excessively strong intensity of the laser light, the polarity of the error signals is selected to as to reduce the current of the element 1. Therefore, the intensity of the laser light 7 can be adjusted in accordance with the modulated input.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source, and more particularly to a highly stable light source having a laser element that changes its wavelength by a magnetic field.

[0002]

2. Description of the Related Art In order to maintain the wavelength of the output light of a laser light source such as a semiconductor laser at a predetermined value, the oscillation wavelength of a laser device is first detected by an etalon or an optical heterodyne. If the oscillation wavelength deviates from the desired value, the deviation, that is, the error is negatively fed back to the laser device, and the oscillation wavelength approaches the desired value. For that purpose, a method of directly modulating the wavelength of the laser device is convenient. Conventionally, a method of modulating the drive current of the laser device or the operating temperature has been used for this purpose. A method of directly modulating the wavelength of a laser device by a magnetic field is known, but no example of applying this method to wavelength stabilization of a laser light source is known.

In the method of modulating the drive current, amplitude modulation is parasitic on the output light, and in the method of modulating the operating temperature, there is a problem that the device is complicated and the allowable modulation frequency is low. for that reason,
It is difficult to obtain a laser light source that produces output light with low noise and stable amplitude.

[0004]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to obtain a highly stable light source which can produce output light with low noise and stable amplitude by using a simple modulating means with high speed and low parasitic amplitude modulation.

[0005]

In order to solve the above-mentioned problems, a highly stable light source of the present invention comprises a laser element such as a laser diode which is energized by a power source to generate output light, and a magnetic field is applied to the laser element. And a wavelength detecting means for inputting the output light and outputting a detection signal according to the wavelength of the output light, and introducing the detection signal into the magnetic field generator to change the magnetic field. And a control means for controlling the detection signal to have a predetermined value.

Further, the highly stable light source of the present invention can be provided with an amplitude modulating means for generating an amplitude modulated output light having a small parasitic wavelength modulation. Furthermore, the highly stable light source of the present invention can be provided with a modulation means for generating wavelength-modulated output light with small parasitic amplitude modulation.

[0007]

In order to generate an amplitude-modulated output light having a small parasitic wavelength modulation, the amplitude modulation means modulates the power source,
The detection signal output from the wavelength detecting means is adjusted so as to suppress parasitic wavelength modulation and then introduced into the magnetic field generator. By modulating the magnetic field with a modulation signal having a frequency higher than the loop band of the control means, it is possible to obtain frequency-modulated output light with small parasitic amplitude modulation. When it is necessary to suppress parasitic amplitude modulation, a signal for the modulation is adjusted and introduced into the power supply.

[0008]

1 is a block diagram of a magnetic field controlled high stability light source according to an embodiment of the present invention. In FIG. 1, a laser element 1 energized by a power source 3 is a semiconductor laser diode and is arranged between magnetic poles of a magnetic circuit 2 having magnetic poles 2a and 2b. The relationship between these laser elements and magnetic poles is known in the YI
It is similar to the G (yttrum iron garnet) tuning circuit. Part of the magnetic poles 2a and 2b can be permanent magnets, and a DC magnetic field in which a magnetic field generated by the current of the coil 2c wound on the magnetic pole 2a is superimposed can be applied to the laser element 1. The coil 2c is driven by the output current of the coil drive circuit 4.

The laser beam 6 output from the laser element 1 splits the output laser beam 7 and the wavelength detecting laser beams 9 and 11 each time it passes through the semi-transparent mirrors 20, 22 and 24, and outputs the laser beam 8. It progresses to 10 and 12. The semitransparent mirror 24 may be omitted depending on the wavelength error generating method of the present invention. The intensity of the laser light 12 is detected by the photodetector 26, and when the switch 52 is closed, the detection output is input to the power supply control circuit 5 as a light intensity output.

The power supply control circuit 5 takes the difference between the light intensity output from the photodetector 26 and the modulation signal input to the modulation signal input terminal 50, integrates and amplifies the difference, and then controls the control terminal M of the power supply 3. Enter as input. The power supply 3 supplies a device current according to the control input to the laser device 1. If the error signal indicates excessive intensity of the laser light, the polarity of the error signal is selected so as to reduce the device current. With the above configuration, a negative feedback loop that forms the intensity of the output laser light 7 in a value according to the modulation input is formed. When the modulation signal input to the modulation signal input terminal 50 is DC, the intensity of the output laser light 7 has a value corresponding to the DC. For other intensity modulation techniques, well-known signal generator techniques are used.
The negative feedback loop can be opened by opening the switch 52.

In the first embodiment for stabilizing the wavelength of the output laser light 7, the terminals s and b of the switch 28 are short-circuited,
Open between terminals s and a. That is, in this embodiment, the semitransparent mirror 2
4 and the etalon spectroscope 25 are unnecessary, and the laser light 10 and the laser light 12 are the same. Laser light for wavelength detection 9
The output of the etalon spectroscope 23 that receives is substantially proportional to the transmittance T of the etalon. FIG. 2 shows the transmittance T for the wavelength λ of the laser light 9 for wavelength detection. The transmittance T is normalized with a peak value of 1.
There are a large number of wavelengths showing the peak value at λn and λn + 1, and these wavelengths λn and λn + 1 are variable.

In this embodiment for stabilizing the wavelength of the output laser beam 7, the error signal generator 30 outputs the weighted difference between the output of the etalon spectroscope 23 and the output of the photodetector 26 as an error signal. For example, if the wavelength of the output laser light 7 is λ = λn + Δλ
When the control is set to, the weighting may be set so that the error signal becomes zero at the output of the etalon spectroscope 23 whose transmittance T is 0.5. As the error signal generator 30, an operational amplifier circuit having a weighted resistance as an input resistance is suitable.

The error signal is converted into a coil current by the coil driving circuit 4. The change in the coil current is translated into a change in the magnetic field applied to the laser element 1, and as a result, the wavelength of the laser light output from this laser element is changed. In this way, the wavelength of the output laser light is λn + Δλ
Negative feedback loop for controlling to. The control of the output laser light is limited to the wavelength range in which the wavelength changes in the vicinity of λn + Δλ and the one-way gain of the negative feedback loop is sufficiently large. Therefore, pre-adjust λn according to the given λ,
It is preferable to satisfy the relationship shown in the schematic diagram 2.

The increase in photon energy in a quantum well laser by applying a magnetic field is larger by applying a magnetic field perpendicular to the quantum well than by applying a magnetic field parallel to the quantum well, and the width of the quantum well is wider. Larger, for example, about 0.1 to 1 meV per 1T. In addition, the sensitivity of the shift amount is higher when the magnetic field strength is higher (eg H. Sakai,
Et. al., Appl. Phys. Lett. Vol. 40, pp 83-85 (198
(See 5)). The change in the magnetic field due to the coil is 0.1 T at most, and therefore, superimposing the variable DC bias magnetic field with the permanent magnet is a good measure for increasing the gain of the negative feedback loop. The variable DC bias magnetic field may mechanically narrow a part of the magnetic path.

FIG. 3 is a diagram showing the relationship between device current and output wavelength of a typical laser diode. As can be seen from the figure, the change in wavelength in one oscillation mode is at most 0.2n.
m. The above energy of 1 meV corresponds to a wavelength change of about 0.8 nm. Commercially available etalon spectrometer (eg Newpor
If SR-200 manufactured by T. Co. is used, the wavelength stability of the wavelength of the output laser light 7 can be set to 0.003 nm or less in the above embodiment.

Since the oscillation wavelength, that is, the wavelength of the output laser beam 7 is increased by the increase of the element current, a signal from the power supply control circuit is introduced into the coil drive circuit 4 via the switch 54 to reduce the applied magnetic field. You may The wavelength of the output laser light can be modulated by opening the switch 54 and introducing the frequency modulation signal from the terminal 56 to the coil drive circuit 4 and adding it to the error signal. In this case, the error signal generator 30
Should sufficiently integrate and output the error signal so as not to follow the wavelength change due to the frequency modulation signal.

In the second embodiment for stabilizing the wavelength of the output laser beam 7, the terminals s and a of the switch 28 are short-circuited,
Open between terminals s and b. That is, in this embodiment, the semitransparent mirror 2
4 and etalon spectrometer 25 are also used. The output of the etalon spectroscope 25 that receives the wavelength detecting laser beam 11 is substantially proportional to the transmittance T of the etalon. FIG. 4 illustrates the transmittance T of the wavelength detecting laser lights 9 and 11 with respect to the wavelength λ. Transmittance T of etalon spectroscopes 23 and 25
Are normalized with the respective peak values being 1, and the wavelengths showing the peak values are separated from each other by 2Δλ.

The error signal generator 30 is an etalon spectroscope 23.
And an output of the etalon spectroscope 25 is output as an error signal. For example, the wavelength of the output laser light 7 is λ =
When the control is set to (λ + Δλ + λ−Δλ) / 2, the weight may be set so that the error signal becomes zero when the wavelength of the output laser light 7 is λ. In this second embodiment, the controllable wavelength range can be made wider than in the first embodiment.

In the above embodiment, it is necessary to avoid interference between the negative feedback loop for stabilizing the wavelength of the output laser light 7 and the negative feedback loop for stabilizing the intensity. However, in the present invention, since the wavelength is tuned by the magnetic field without changing the intensity, there is almost no such interference.
Further, as is clear from the above description, the spectroscope is not limited to the etalon spectroscope, and various spectroscopes can be used.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an outline of a configuration of an embodiment of the present invention.

FIG. 2 is a graph showing wavelength characteristics of transmittance of a spectroscope used in the embodiment of FIG.

FIG. 3 is a graph showing a device current characteristic of an oscillation wavelength of a laser diode.

FIG. 4 is a graph showing wavelength characteristics of transmittance of two spectroscopes used in the embodiment of FIG. 1 at the same time.

[Explanation of symbols]

 1 Laser Element 2 Magnetic Circuit 3 Power Supply 4 (Coil) Drive Circuit 5 Power Supply Control Circuit 6, 8, 10, 12 Laser Light 7 Output Laser Light 9, 11 Wavelength Detection Laser Light 20, 22, 24 Semitransparent Mirror 23, 25 Etalon spectroscope 26 Photodetector 28, 52, 54 Switch 30 Error signal generator 50 Modulation signal input terminal 56 (Frequency modulation signal input) terminal

Claims (6)

[Claims] 1. A laser element which is energized by a power source to generate output light, a magnetic field generator for applying a magnetic field to the laser element, and the output light which is input to a wavelength of the output light. A magnetic field consisting of a wavelength detection means for outputting a detection signal according to the above, and a control means for introducing the detection signal into the magnetic field generator and changing the magnetic field so that the detection signal has a predetermined value. Controlled highly stable light source. 2. The magnetic field controlled highly stable light source according to claim 1, wherein said wavelength detecting means is an etalon spectroscope. 3. The magnetic field controlled high stability light source according to claim 1 or 2, further comprising an amplitude modulation means connected to the power source for generating an amplitude modulation signal for amplitude modulating the output light. 4. A magnetic field controlled high stability light source according to claim 3, further comprising means for introducing said amplitude modulated signal into said magnetic field generator. 5. The magnetic field controlled high stability light source according to claim 1, further comprising frequency modulation means connected to said magnetic field generator for generating a frequency modulation signal for modulating said magnetic field. 6. The magnetic field controlled high stability light source according to claim 1, wherein the magnetic field has a predetermined DC magnetic field which is not zero.