JPH088392B2 - Variable wavelength light source - Google Patents

Description

TECHNICAL FIELD The present invention relates to an improvement of a variable wavelength light source which is used in an optical spectrum analyzer or the like and enables precise wavelength measurement.

<< Prior Art >> Conventionally, when measuring a wavelength characteristic or a spectral characteristic using an optical spectrum analyzer, a spectroscope, or the like, a light source serving as a wavelength reference is required to improve accuracy.

<< Problems to be Solved by the Invention >> However, there is a drawback that an error becomes large when a band away from the wavelength of the reference wavelength light source is measured.

Also, if a variable wavelength light source is used as the reference wavelength light source, the wavelength range can be widened, but it is not easy to accurately associate the input of the variable wavelength light source with the oscillation wavelength.

The present invention has been made to solve such a problem, and an object thereof is to realize a variable wavelength light source that enables highly accurate wavelength measurement over a wide band.

<< Means for Solving the Problems >> The present invention relates to a variable wavelength light source in which the wavelength of output light changes in response to an input signal, and is characterized by a reference wavelength light source and a reference wavelength light source. A phase modulator that phase-modulates output light to generate subcarrier light, a laser light source, an optical unit that combines a part of the output light of the laser light source and the output light of the phase modulator, and the optical device. A light-receiving element that receives the combined light from the means and generates an electric signal of the beat frequency, and a control means that controls the output frequency of the laser light source so that the beat frequency from the light-receiving element becomes constant. In addition, the output wavelength of the laser light source is variable according to the modulation frequency of the phase modulator.

<< Function >> If the wavelength of the laser light source is controlled to one of the sub-carrier lights generated by the phase modulator and the modulation frequency of the phase modulator is changed, the wavelength of the sub-carrier light changes. Also changes.

<Example> The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a configuration block diagram showing an embodiment of a variable wavelength light source according to the present invention. In the figure, the optical path is represented by a band-shaped signal line to distinguish it from an electric signal. 1 is, for example, C _s or R _b
A reference wavelength light source having a very stable wavelength using a semiconductor laser whose wavelength is controlled by an atomic absorption line, etc., 2 is a phase modulator for phase-modulating the output light of the reference wavelength light source 1 by phase modulation, 3 Is an input terminal to which an input frequency signal is applied, 4 is an amplifier that amplifies the input electric signal from this input terminal 3 and frequency-modulates the phase modulator 2 with its output, and 5 is the output light of the phase modulator 2. An incident beam splitter, 6 is a light receiving element for injecting the output light of the beam splitter 5, 7
Is a control circuit for inputting the electric output of the light receiving element 6, 8 is a semiconductor laser which constitutes a laser light source and receives the output of the control circuit 7, 9 is a beam splitter which makes the output light of the semiconductor laser 8 incident, 11 is this beam splitter 9
It is a mirror that reflects the reflected light of and makes it enter the beam splitter 5. Beam splitter 5,9 and mirror 10,
Reference numeral 11 constitutes an optical means for multiplexing a part of the output light of the laser light source 8 and the output light of the phase modulator 2. Various types such as a bulk type and a waveguide type can be used as the phase modulator 2, but the waveguide type is superior in that a large number of subcarriers are generated with low power. As the semiconductor laser 8, a semiconductor laser having a spectrum width as narrow as possible and less mode hopping, such as a DFB laser, is used.

FIG. 2 is a configuration block diagram showing a specific configuration example of the reference wavelength light source 1. In the figure, 11 is a semiconductor laser, 12 is a beam splitter on which the output light of this semiconductor laser 11 is incident, 13 is an absorption cell in which a standard substance on which the reflected light of this beam splitter 12 is incident is enclosed, and 14 is this absorption cell 13 , A lock-in amplifier for controlling the current of the semiconductor laser 11 with an input corresponding to the electric output of the light-receiving element 14, and 16 for controlling the current of the semiconductor laser 11 by frequency. An oscillator that modulates and supplies the phase detection frequency of the lock-in amplifier 15. The transmitted light of the beam splitter 12 becomes the output light of this reference wavelength light source. As the standard substance, any substance such as C _s , R _b , NH ₃ , and H ₂ O can be used. The output light of the semiconductor laser 11 is reflected by the beam splitter 12, enters the absorption cell 13, and is absorbed by the standard substance in the absorption cell 13. The absorption amount is detected by the light receiving element 14, and is fed back to the current of the semiconductor laser 11 via the lock-in amplifier 15. Since the output wavelength of the semiconductor laser 11 is locked to the absorption spectrum line of the standard substance, a highly stable and highly accurate reference wavelength light source can be realized.

FIG. 3 is a configuration block diagram showing a specific example of the control circuit 7. Reference numeral 71 is a comparator for inputting an electric output from the light receiving element 6, 72 is an oscillator for generating a setting frequency, 73 is a phase comparator for inputting the output of the comparator 71 and the output of the oscillator 72, and 74 is a phase comparator. The servo amplifier receives the output of the phase comparator 73. The output frequency of the light receiving element 6 is compared with the oscillation frequency of the oscillator 72 by the phase comparator, and the servo amplifier 74 controls the output frequency of the semiconductor laser 8 so as to eliminate the difference between the frequencies.

The operation of the variable wavelength light source configured as described above will be described below. The light having the frequency f _R output from the reference wavelength light source 1 is phase-modulated at the modulation frequency f _m in the phase modulator 2 to generate a large number of subcarriers (subcarriers) as shown in FIG. A part of the output light of the semiconductor laser 8 is reflected by the beam splitter 9, is combined with the output light of the phase modulator 2 in the beam splitter 5 via the mirrors 10 and 11, and is received by the light receiving element 6 as an electric signal of the beat frequency. To occur. As described above, the control circuit 7 controls the semiconductor laser 8 so that the signal frequency from the light receiving element 6 becomes constant. As a result, the frequency of the light output from the semiconductor laser 8 after passing through the beam splitter 9 has a certain frequency difference from the output light of the specific wavelength of the phase modulator 2. When the frequency f _o of the semiconductor laser 8 is locked to the Nth subcarrier contained in the output light from the phase modulator 2, f _o = f _R + Nf _m (1) Therefore, the input frequency f _m The output frequency f _o can be changed accordingly.

According to the variable wavelength light source having such a configuration, the stability of the output frequency is determined by the stability of the reference wavelength light source and the stability of the input frequency of the phase modulator, and thus the stability is excellent.

Further, since the output frequency is obtained by adding the integer multiple of the input frequency of the phase modulator to the frequency of the reference wavelength light source (the one having a constant frequency difference), the output frequency is highly accurate.

In the above embodiment, instead of the amplifier 4, VCO (V
It is also possible to use voltage as an input signal instead of frequency by using an oltage controlled oscillator (oscillator) or a synthesizer. F _m input frequency f _m of the phase modulator 2 in this case is the input voltage V _in
= G · V _in , then f _o = f _R + N · G · V _in (2), and the output frequency can be changed according to the input voltage.

FIG. 5 is a configuration block diagram showing a modified example of the present invention. The lock-in amplifier 75 is used as the control means, and the second
If another frequency modulation of the frequency f _p is applied to the phase modulator 2 by the oscillator 21 of, and is used as the phase detection frequency of the lock-in amplifier 75, the output frequency of the semiconductor laser 8 becomes the center frequency of the subcarrier without the offset frequency. You can control. In this case, the frequency modulation is performed by the reference wavelength light source 1.
Can also be done in.

In each of the above embodiments, an optical fiber coupler or an optical fiber may be used as the optical means instead of the beam splitter or the mirror to reduce the size.

The method of FIG. 2 shown as one embodiment of the reference wavelength light source 1 is called a linear absorption method, and the absorption spectrum becomes relatively thick due to Doppler shift, but the saturated absorption method (Hori, Kadota, Kitano, Yabusaki, Ogawa) : Frequency stabilization of a semiconductor laser using saturated absorption spectroscopy, and the absorption line of the hyperfine structure hidden by Doppler shift is detected by the technical report of OQE82-116), and the oscillation wavelength of the semiconductor laser 11 is locked to this. If this is done, it can be made even more stable.

<< Advantages of the Invention >> As described above, according to the present invention, it is possible to realize a variable wavelength light source that enables highly accurate wavelength measurement over a wide band with a simple configuration.

[Brief description of drawings]

FIG. 1 is a structural block diagram showing an embodiment of a variable wavelength light source according to the present invention, FIG. 2 is a structural block diagram showing the details of a reference wavelength light source 1 of the apparatus of FIG. 1, and FIG. 3 is an apparatus of FIG. 4 is a block diagram showing the details of the control circuit 7, FIG. 4 is a diagram showing a frequency spectrum of the output of the phase modulator 2, and FIG. 5 is a block diagram showing a modified example of the apparatus shown in FIG. 1 ... Reference wavelength light source, 2 ... Phase modulator, 5, 9, 10, 11 ...
... Optical means, 6 ... Light receiving element, 7,75 ... Control means, 8 ...
… Laser light source, f _m … Modulation frequency.

Claims (3)

[Claims] 1. A variable wavelength light source in which the wavelength of output light changes in response to an input signal, and a reference wavelength light source, and a phase modulator for phase modulating the output light of the reference wavelength light source to generate subcarrier light. A laser light source, an optical means for combining a part of the output light of the laser light source and the output light of the phase modulator, and the combined light from the optical means for receiving the electric power of the beat frequency. A light receiving element for generating a signal and a control means for controlling the output frequency of the laser light source so that the beat frequency from the light receiving element is constant, and the output of the laser light source corresponding to the modulation frequency of the phase modulator Variable wavelength light source characterized in that the wavelength is variable. 2. The variable wavelength light source according to claim 1, wherein a semiconductor laser whose wavelength is controlled by an atomic absorption line of C _s or R _b is used as the reference wavelength light source. 3. The variable wavelength light source according to claim 1, wherein a semiconductor laser is used as the laser light source, and the output frequency is controlled by operating the current or temperature thereof.