JPH03137526A - Measuring apparatus of spectral width of laser - Google Patents

Abstract

PURPOSE:To measure a spectral width of a laser light by converting a frequency variation of the laser light into a variation of the intensity of light by the frequency discrimination characteristic of a Fabry-Perot etalon and by catching this variation as a variation of an electric output by a detector. CONSTITUTION:An output light of a laser 1 is passed through a collimator lens 2 and an isolator 3 and made to branch by a splitter 4. A light transmitted through a sweeping-type Fabry-Perot etalon 5 is detected by a detector 6, while a reflected light is detected by a detector 7. When the etalon 5 is controlled so that an output ratio between the two detectors be fixed, a slanting part of the resonance characteristic of the etalon 5 is always is accord with the frequency of a laser light to be measured. Accordingly, a spectral width of the laser light to be measured, i.e. a phase fluctuation thereof, is converted into an amplitude fluctuation by the etalon 5 and outputted from the detector 6. A DC part of a fluctuation signal being cut, the signal is inputted to a voltage value histogram display unit 10 and the spectral width is displayed.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to improving the characteristics of a device for measuring the spectral width of a laser.

<Prior Art> In order to measure the spectral width of a laser, the following various methods have been used in the past.

a. Delayed self-heterodyne method (homodyne method) After the laser beam to be measured is branched and passed through a delay optical fiber and a tone optical element, the combined light is detected and measured with a spectrum analyzer.

b. Scanning Fabry-Perot type laser beam to be measured is incident on a sweep type Fabry-Perot etalon, the transmitted light is detected and displayed on a CRT using a sweep signal.

C. Beat method The reference frequency laser beam and the laser beam to be measured are combined, the beat signal is detected, and it is measured with a spectrum analyzer.

<Problems to be Solved by the Invention> However, each of the above methods has the following problems. That is, the delayed self-heterodyne method of a requires a considerable length of expensive optical fiber in order to improve the resolution. Furthermore, the scanning Fabry-Perot type shown in b requires a Fabry-Perot etalon with a high Q to improve resolution. C
The Noto-1~ method requires a laser with the same frequency and narrow spectrum as the reference frequency laser beam.

The present invention has been made to solve these problems, and an object of the present invention is to realize a high-resolution laser spectral width measuring device with a simple configuration and inexpensive parts.

Means for Solving the Problems> The laser spectral width measuring device according to the present invention includes a light branching means for branching a laser beam to be measured into two, and a sweep type fabricator into which the output light of one of the light branching means is incident. a Perot etalon, a first photodetector that detects the transmitted light of the Fabry-Perot etalon, a second photodetector that receives the other output light of the optical branching means; a control means for controlling the ratio of the output of the second photodetector to be equal to a predetermined set value; 1
The first photodetector detects the laser beam, and the spectral width of the laser beam to be measured is measured from the amplitude fluctuation of the first photodetector.

<Operation> Frequency fluctuations of the laser beam to be measured are converted into fluctuations in the output light intensity by the frequency discrimination characteristics of the Fabry-Perot etalon.
A first photodetector converts it into a variation in electrical output. The spectral width of the laser beam to be measured can be measured from this variation in electrical output.

<Example> The present invention will be explained in detail below using the drawings.

FIG. 1 is a block diagram showing an embodiment of a laser spectral width measuring device according to the present invention. 1 is a laser to be measured, 2 is a collimating lens that converts the output light of the laser to be measured 1 into parallel light, 3 is an optical isolator for preventing return light through which the emitted light from the collimating lens 2 passes, and 4 is an output of the optical isolator 3. Beam splitter that splits the emitted light into two directions, 5
51 is a PZT that changes the interval between the mirrors of the swept Fabry-Perot etalon 5; and 6 is a first type Fabry-Perot etalon that receives the light transmitted through the swept Fabry-Perot etalon 5. A photodetector 7 is a second photodetector into which the reflected light from the beam sinter 4 is incident, and a control unit 8 is a first photodetector. A control unit that drives the PZT 51 so that the ratio of the output of the second photodetector 6.7 becomes equal to the set input, 9 is a capacitor that cuts the DC component of the output of the photodetector 6, and 10 is the AC of the capacitor 9. This is a voltage value histogram display that displays the output as a histogram.

The operation of the laser spectral width measuring device having the above configuration will be explained below with reference to FIGS. 1 and 2. The output light of the laser to be measured 1 enters the beam splitter 4 via the collimating lens 2 and the optical isolator 3 and is split into two.The transmitted light of the swept Fabry-Perot etalon 5 is detected by the photodetector 6 and reflected. Light is detected by a photodetector 7. Since the control unit 8 controls the etalon 5 so that the output ratio of the photodetector 6 and the photodetector 7 is constant, the slope portion of the resonance characteristic of the etalon 5 is always exposed, as shown in FIG. The frequency will match the measurement laser beam frequency. Therefore, the spectral width or phase fluctuation of the laser beam to be measured is converted into amplitude fluctuation by the etalon 5 and output from the photodetector 6. After the DC portion of this amplitude fluctuation signal is cut, it is input to the voltage value histogram display 10, a histogram of which is taken, and then the subelum width of the laser beam to be measured is displayed. Note that a digital oscilloscope can be used as the histogram display, but it can also be configured with a high-speed AD converter 11 and a computer 12 as shown in FIG. Etalon 5 on computer 12
We also linearize the nonlinearity in the shaded area in the resonance characteristics.

For example, as etalon 5, FSR2GHz (interval 25m
m), finesse is 20, the peak width of the resonance characteristic is 10
0 MHz, and the spectral width of a normal semiconductor laser, about 20 MHz, can be sufficiently constant in the linear portion of the etalon characteristics. Also, as 11 in Figure 3, the 8-bit A
If a D converter is used, there will be a resolution of 400 kHz out of 100 MHz/256.

According to the laser spectral width measurement device having such a configuration, neither a delay optical fiber nor a high finesse etalon is required, resulting in low cost.

Furthermore, very high resolution measurements can be made using inexpensive etalons.

Note that in the above embodiments, any resonator whose resonant characteristics (ie, resonant frequency) can be controlled can be used instead of the swept Fabry-Perot etalon, such as a ring resonator or a Meigelson interferometer.

Moreover, an optical fiber can also be used for inputting the light to be measured. In this case, a fiber coupler can also be used instead of a beam splitter.

Further, the FSR and finesse may be changed according to the spectral width of the laser to be measured.

Further, the half width of the frequency fluctuation of the laser beam to be measured can also be directly determined from the output of the photodetector 6.

<Effects of the Invention> As described above, according to the present invention, a high-resolution laser spectral width measuring device can be realized with a simple configuration and inexpensive parts.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the laser spectral width measuring device according to the present invention, FIG. 2 is an explanatory diagram of the operation of the device shown in FIG. 1, and FIG. 3 is the voltage value histogram display 10 of FIG. FIG. 2 is a partial configuration block diagram showing a specific example. 1... Laser to be measured, 4... Optical branching means, 5...
Sweep type Fabry-Perot etalon, 6... first photodetector, 7... second photodetector, 8... control means.

Claims (1)

[Claims] A light branching means for branching the laser beam to be measured into two, a swept Fabry-Perot etalon into which the output light of one of the light branching means is incident, and a first one for detecting the transmitted light of the Fabry-Perot etalon. control so that the ratio of the outputs of the photodetector, the second photodetector into which the other output light of the light branching means is incident, and the first and second photodetectors is equal to a predetermined set value; A first photodetector detects the output light intensity of the Fabry-Perot etalon, which changes in response to frequency fluctuations of the laser beam to be measured, and detects the output light intensity from the amplitude fluctuations of the first photodetector. A laser spectral width measurement device characterized in that it is configured to measure the spectral width of a measurement laser beam.