JPS61165627A - Spectrum line width meter of laser beam - Google Patents

Abstract

PURPOSE:To measure spectrum line width of a laser beam source, by coupling a part of beam from the specimen laser beam source to a light detecting means and the balance to a circulating loop, shifting the phase of a beam of light propagating the circulating loop, and bringing the light beam coupled to the light detecting means into resonance. CONSTITUTION:A beam of light emitted from a laser beam source 13 is admitted to an optical fiber 10 of single mode, and a part is divided into a light detector 6 by an optical fiber directional coupler 8 and the balance into a circulating loop 100 of the single-mode optical fiber 10. The beam of light which has circulated through the loop 100 is partly propagated to the detector 6 by the coupler 8 again and the balance to the loop 100 again. By changing the phase of the light beam circulating through the loop 100 by a phase modulating apparatus 11, a beam of light in which multiple interference has been caused is detected by the detector 6. A spectrum line width of the light source 13 can be measured in response to a resonance output of the detector 6.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a spectral linewidth measuring device for a laser light source, and particularly to a laser light source with a wide oscillation spectral linewidth, such as a semiconductor laser (hereinafter abbreviated as "D"). The present invention relates to an apparatus for measuring the spectral linewidth of.

[Prior Art] FIG. 3 shows, for example, E controller+ics L
ettersVol, 16. No. 16 pp.
630-631 is a configuration diagram of a conventional LD oscillation spectrum linewidth measuring device. First, the configuration of this device will be explained. In the figure, LD light W! In front of A1, a lens 2a, a beam splitter 3a, and a lens 2b are provided in this order on the same optical axis. In front of the lens 2b, one end of a long (usually 1-2 mm) single mode optical fiber 5 is provided on the optical axis, and this fiber forms a plurality of circulation loops 50. A lens 2C is provided in front of the other end of the single mode optical fiber 5, and a beam splitter 3b, a photodetector 6. Spectrum analyzers 7 are provided in this order. Further, an ultrasonic optical modulator 4 is provided between the beam splitter 3a and the beam splitter 3b.

Next, the operation of this device will be explained. LD light 1Ti1
The emitted light is parallelized by a lens 2a and split into two by a beam splitter 3a. One of the lights is input into a long single mode optical fiber 5 by a lens 2b, and the other light is subjected to ultrasonic light modulation. The frequency is shifted by fb by the unit 4. The light exiting the single mode optical fiber 5 and the light passing through the ultrasonic optical modulator 4 are combined by a beam splitter 3b and interfere with each other on a photodetector 6. The light that has circulated through the circulation roof 50 of the single mode optical fiber 5 is delayed in phase by the propagation time τ of this optical fiber than the light that has passed through the ultrasonic optical modulator 4. Since a long single mode optical fiber 5 is used,
The propagation time τ is longer than the coherence time of the LD light source 1. Therefore, although these two lights do not interfere in the usual sense, a correlated signal of these two lights appears at the output of the photodetector 6. By analyzing this correlation signal with the spectrum analyzer 7, the spectrum of the LD light source 1 can be determined, and from this, the spectral linewidth can be determined.

[Problems to be Solved by the Invention] The conventional LD emission spectral linewidth measurement device is configured as described above, so it uses a long single mode optical fiber, an ultrasonic optical modulator, and multiple optical components. The measurement system was extremely complex and required a large amount of space. Furthermore, the modulation frequency r of the ultrasonic optical modulator is several tens of MH2
Because of the above, a broadband photodetector is required, which poses a problem in that it is extremely difficult to measure the spectral line width.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a spectral linewidth measurement device for a laser light source that is compact and capable of easily measuring the spectral linewidth of a laser light source.

[Means for Solving the Problems] A spectral linewidth measuring device for a laser light source according to the present invention configures an optical fiber ring interference system with an optical fiber including a circulation loop, an optical coupling means, and a phase shift means, and A laser light source to be measured is coupled to one end of the fiber, and a light detection means is coupled to the other end, the light from the laser light source to be measured is coupled to the optical fiber, a part of this light is coupled to the light detection means, and the remainder is coupled to the light detection means. is coupled to the circulation loop, a part of the light that has circulated through the circulation loop once is coupled to the optical coupling means, and the remainder is coupled to this circulation loop again for recirculation, and the light that has been circulated twice further, 3 The same operation as above is repeated sequentially for the circulating light, etc., and the phase shift means shifts the phase of the light propagating through the circulation loop, causing the light coupled to the light detection means to resonate, and the light to be coupled to the light detection means The spectral linewidth of the laser source is measured in response to the resonant output of the laser.

[Function] In this invention, a part of the light from the laser light source to be measured, a part of the light that has circulated once through the circular loop, and a part of the light that has circulated twice through the circular loop are connected to each other by the optical coupling means. ,...
are sequentially coupled to the photodetection means, and the phase shifting means shifts the phase of the light propagating through the circulation loop to cause the light coupled to the photodetection means to resonate.

[Example] Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a spectral linewidth measuring device for a laser light source, which is an embodiment of the present invention. First, the configuration of this device will be explained. In the figure, a laser light source 13 to be measured is coupled to one end of a short single-mode optical fiber 10, which forms a circulation loop 100 after being coupled to an optical fiber directional coupler i. and is again coupled to the optical fiber directional coupler 8. The other end of the single mode optical fiber 10 is coupled to a photodetector 6 and an amplifier 9
It is connected to the output terminal 14 via. The optical fiber directional coupler 8 couples a portion of the light from the laser beam 8113 to the photodetector 6 and the remainder to the circulation loop 100 of the single mode optical fiber 10, through which it circulates. A portion of the light is coupled into the photodetector 6 and the remainder is coupled back into the circulation loop 100. The photodetector 6 receives light propagating through the single mode optical fiber 10 and photoelectrically converts it.

A phase shifter 3m2111 is provided for the circulation loop 100. The phase modulator 11 includes a piezoelectric element, around which a portion of the single mode optical fiber 10 is wound.

A signal generator 12 is connected between the phase modulator 1] and ground. The single mode optical fiber 10, the optical fiber directional coupler 8, and the phase modulation B11 constitute an optical fiber ring interference system. Signal generator 12 generates a triangular wave signal. The piezoelectric element of phase modulator 11 changes its dimensions in response to the triangular wave signal, thereby changing the optical path length of single mode optical fiber 10 and shifting the phase of the light propagating through circulation loop 100. The other end of the single mode optical fiber 10 is coupled to a photodetector 6, which is coupled to an amplifier 9. It is connected via the output terminal 14 to a measuring means, for example an oscilloscope (not shown).

Next, the operation of this device will be explained. Laser light source 13
The light emitted from the is coupled into a single mode optical fiber 10, and a part of it is sent to a photodetector 6 by an optical fiber directional coupler 8, and the rest is sent to a circulation loop 100 of the single mode optical fiber 10. Can be divided. A portion of the light that has circulated through the circulation loop 100 is propagated again to the photodetector 6 by the optical fiber directional coupler 8, and the rest is propagated through the circulation loop 100 again. In this way, the light reaching the photodetector 6 has been circulated once or twice through the circulation loop 100 of the single mode optical fiber 10.

It is the sum of the three cycles of... Therefore, in the photodetector 6, multiple interference of these lights occurs and when the phases are aligned, a sharp resonance occurs. If we change the phase of the light propagating in the circulation loop 100 with the phase modulator 11 and observe the output of the amplifier 9 with an oscilloscope, we get
Sharp resonance can be seen when the phase satisfies the resonance conditions. Figure 2 shows this situation.

Finesse F is an expression of the resonance characteristics of such an optical fiber ring interference system. The finesse F is the interval between the resonance peaks in Figure 2 (Free3 pectral ra
nae: FSR) and the half width of the resonance peak Δri
Using this, it is defined as F-FSR/Δf, (1). The larger the value of finesse F, the sharper the resonance of the optical fiber ring interference system.

Here, assuming that the resonance characteristics of this interference system are sufficiently sharp,
It is approximated by the Lorentz distribution n as shown in the following equation.

φ, (r) - (ξ/π)/(ξ2+(f-fd)')...(2) However, with ξ-c/(2FnL)-Δf, /2te,
f is the frequency of light, C is the speed of light, n is the refractive index of the single mode optical fiber 10, L is the length of the circulation loop 100 of the single mode optical fiber 10, and to is the resonant frequency. Further, if the oscillation spectrum of the laser beam 11113 to be measured has a Lorentz distribution, the spectrum is expressed by the following equation.

φt<r>-<γ/π)/(γ2+72)...(3)
However, γ-Δ'L/2, and Δrb is the half-width of the spectrum of laser light 1 [13]. The resonance characteristics when the light is incident on an optical fiber ring interference system having resonance characteristics can be obtained by correlating the equations (2) and (3).

That is, φ<t > −5p, <δ)・φL(δ−f)dδ−(
(ξ+γ)/π)·[1/((ξ +7)'+(f-ro)')] -<4).

From this equation, the finesse F- of this interference system in consideration of the spectrum spread of the laser light source 13 to be measured is expressed by the following equation.

F−-(c/nL)/2(ξ+γ) −c F/ (Fn LΔh+c) ・(
5) From equation (5), if the finesse F of this interference system is known in advance, by measuring F', the half-width Δ'' of the spectrum of the laser beam 8113 to be measured can be found. The finesse F can be determined from the resonance characteristics when a laser light source with a very narrow spectral linewidth, such as a He-Ne laser, is used.

In addition, in the above-mentioned practical IIIPjI4, the laser light source and the single mode optical fiber are directly coupled, but if the reflected light from the end face of the single mode optical fiber becomes a problem, the laser light source and the single mode optical fiber are coupled directly. It is necessary to insert an isolake in between to eliminate light returning to the laser light source.

Further, although a normal single mode optical fiber is used in the above embodiment, a polarization maintaining optical fiber may also be used.

[Effects of the Invention] As described above, according to the present invention, an optical fiber ring interference system? Since the spectral line width of the laser light source is measured from the finesse of the
Furthermore, the spectral linewidth of a laser light source can be measured easily and with high precision.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a spectral linewidth measuring device for a laser light source, which is an embodiment of the present invention. FIG. 2 is a diagram showing an oscilloscope observation of the amplifier output when resonance conditions are satisfied in the present invention. FIG. 3 is a configuration diagram of a conventional spectral linewidth measuring device for a laser light source. In the figure, 1 is a semiconductor laser (LD>light source, 2a, 2
b, 2c are lenses, 3a, 3b are beam splitters, 4
is an ultrasonic optical modulator, 5.10 is a single mode optical fiber,
50.100 is a circulation loop, 6 is a photodetector, 7 is a spectrum analyzer, 8 is an optical fiber directional coupler, 9 is an amplifier, 11 is a phase shifter: IIA, 12 is a signal generator, 13
is a laser light source. Note that the same reference numerals in each figure indicate the same or corresponding parts. Agent Masu Oiwa Oishi 2 figure 4L practice 3rd jO: Prayer Milk Leah + Procedural amendment (voluntary)

Claims (1)

What is claimed is: a laser light source whose spectral linewidth is to be measured; an optical fiber that includes a circular loop, one end of which is coupled to the laser light source and that propagates light from the laser light source; a photodetector coupled to the other end of the optical fiber for receiving and photoelectrically converting the light propagating through the optical fiber; and a photodetector provided for the optical fiber for coupling a part of the light from the laser light source to the photodetector. optical coupling means for coupling the remainder to said circulation loop, for coupling a portion of the light from said circulation loop to said light detection means and for coupling the remainder back to said circulation loop; phase shifting means for shifting the phase of the light propagating through the circulation loop so as to cause the light coupled to the light detection means to resonate; in response to a resonant output from the light detection means; A spectral linewidth measuring device for a laser light source, comprising: a measuring means for measuring the spectral linewidth of the laser light source.