JPH01148926A - Optical spectrum analyser - Google Patents

Abstract

PURPOSE:To analyze a weak spectrum, by adding a separate interference route separate from one for measuring use and obtaining a narrow interferogram to form the measuring start trigger signal of light to be measured and preventing the generation of time jitter. CONSTITUTION:The light of a white light source 10 is incident to an interferomater as start triggering beam 1' while the beam 1' is separated into two beams by a half mirror 2' and both beams are reflected by a fixed mirror 31 and a double-side movable mirror 4' to interfere with each other through the mirror 2'. The inteferogram obtained herein is inputted to a start trigger pulse generator 14 through a photodetector 5' and an amplifier 6', and the output pulse of the generator 14 starts the sampling of an A/D converter 7. A driver 9 drives the movable mirror 4 to perform reciprocating scanning. By this constitution, even if there is mechanical backlash in the driver 9, the time difference between the interferograms detected by photodetectors 5, 5' becomes constant and the interferograms are simply added and integrated at every scanning of the movable mirror to prevent the generation of jitter and a weak spectrum can be accurately measured.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical spectrum analyzer that performs spectrum analysis of weak light with high accuracy.

(Prior Art) A conventional optical spectrum analyzer using Fourier spectroscopy has a configuration as shown in FIG.

In FIG. 6, the light to be measured 1 is split into two light beams 1a and lb by a half mirror 2, the light beam 1a is reflected by a fixed mirror 3, the light beam 1b is reflected by a movable mirror 4, and each beam is reflected by a half mirror 2 again. The light passes through the beam and enters the photoreceiver 5 in an overlapping manner. At this time, interference occurs between the two light beams due to the optical path difference between the light beams 1a and lb, and an interferogram as shown in FIG. 7 is observed at the output end of the amplifier 6. This is A/
When the signal is A/D converted by a D converter 7 and passed through a fast Fourier transform device 8, a spectrum is measured as shown in FIG.

In order to accurately observe even the very weak portion of the optical spectrum of the light to be measured 1, an averaging method can be considered. In order to perform averaging on the time axis, the movable mirror is moved repeatedly,
Interferograms are acquired and averaged for each movement cycle. However, in the interferogram shown in FIG. 7, due to "backlash" in the mechanism, jitter of about ±1 μm inevitably occurs due to repeated operations. This situation is shown in FIGS. 9(a) and 9(b). (a) shows the optical power when scanning is performed only once, and Φ) shows the overwritten optical power when scanning is performed 10 times. Naturally, if a jittered waveform is subjected to Fourier transformation, accurate spectral measurements cannot be obtained. As described above, conventional optical spectrum analyzers using Fourier spectroscopy have difficulty in averaging over time, making it difficult to analyze weak spectra.

(Problems to be Solved by the Invention) The present invention has an optical needle configured so that even if there is "backlash" in the movable mirror drive mechanism, jitter does not occur in the interferogram, and the optical needle is The object of the present invention is to provide an optical spectrum analyzer that can perform a weak spectrum analysis.

(Means for Solving the Problems) The present invention additionally configures an interference path separate from the one for measurement, and inputs a signal from a light source with a wide spectrum such as white light into the interference path, and the resulting narrow The interferogram is used as a measurement start trigger signal for the light to be measured. At this time, the movable mirror is mechanically integrated with the measurement movable mirror and driven by the same driver, and the trigger interferogram and measurement light interferogram are Prevent time jitter from occurring between.

The present invention differs from conventional techniques in that accurate time axis averaging is possible.

(Embodiment) FIG. 1 is a configuration diagram of a first embodiment of the present invention,
A second interferometer is incorporated for generating the D-conversion start trigger signal. The light from the white light source 10 for the start trigger is narrowed down by a lens 11, an optical fiber 12, and a collimator 13.
After that, the start trigger light beam 1' is injected into the interference needle. The half mirror 2' divides the beam into two beams 1'a and 1'b, and the beam 1/a is reflected by the fixed mirror 3'.
The beams l'b are reflected by the double-sided movable mirror 4', each pass through the half mirror 2' again and interfere with each other, and the interferogram is photoelectrically converted by the photoreceiver 5'. 6' is an amplifier. 14
is a start trigger pulse generator that shapes the waveform.
It is composed of a comparator, a multi-pipe rake, etc., and a step pulse is outputted from the input from the amplifier 6', which serves as a start trigger and causes the A/D converter 7 to start sampling. Reference numeral 9 denotes a driving body for the double-sided movable mirror 4', which reciprocates the double-sided movable mirror 4' in the left-right direction.

With this configuration, the time difference between the interferograms detected by the light receiver 5.5' remains constant even if there is mechanical "backlash" in the driving body 9. When white light is used as a light source, the output interferogram becomes pulsed in a narrow time range, so if this is used as the measurement start trigger for the light to be measured, an ideal trigger signal can be obtained.

When a device with good coherence, such as a semiconductor laser, is used instead of the white light source 10, the interferogram shown in FIG. 2(b) differs from the white light interferogram shown in FIG. 2(a). This results in ink-ferrograms that interfere over a wide range of axes, making the timing of the start trigger signal uncertain. Ideally, a narrow, single-shot interferogram is good. The double-sided movable mirror 4' is moved by the movable mirror driver 9,
A start trigger pulse is generated only once in one scan only when moving to a certain position (point of optical path difference O of the second interferometer).

Here, a fixed mirror 3', a half mirror 2', a light receiver 5'
If they can be made to move slightly in the direction of the incident optical axis, the point of optical path difference O between the light beams 1'a and 1'b, that is, the interferogram peak position, can be varied, and the trigger signal can be moved on the time axis. can.

In other words, interferogram measurement (observation) of the light to be measured
The area (window) can be set arbitrarily.

In this way, the double-sided movable mirror 4 of the second interferometer has an optical path difference of 0.
Since sampling of the A/D converter is performed only for the position ', when the double-sided movable mirror 4' is repeatedly scanned, backlash and hysteresis due to the mechanism of the movable mirror driver 9 may occur. Also, the interfnibram of the light to be measured can always be measured from a constant optical path difference position. Therefore, a waveform completely free of jitter can be obtained, and addition and integration can be performed as is. FIG. 3(b) shows an example of the power averaged 1024 times. The power without addition is shown in Figure 3 (
Shown in a). The noise level without addition is -72d
Bm/μm, but after averaging -80dBm/μm
It has improved to below m.

FIG. 4 is a configuration diagram of a second embodiment of the present invention, which includes 1.
la and lb are the light fluxes that constitute the thread path of the light to be measured, 1'
+ 1'a + 1'b is a light beam forming a start trigger optical path, 1'', 1''a, 1''b is a light beam forming a data sampling trigger signal optical path,
A HeNe laser or a semiconductor laser is used as a light source. 4" is a movable mirror consisting of two mirror surfaces 4.4 with a step δ. 5.5" and 5" are photoelectric converters for the interferogram of each luminous flux, and 6.6' and 6" are amplifier, 15
is a sampling trigger waveform shaping circuit, which is composed of a rectangular wave shaping circuit such as a Schmitt trigger circuit.

The output of the start trigger pulse generator 14 and the output of the sampling trigger waveform shaping circuit 15 are connected to an AND circuit 16, and the external trigger type A/D converter 7' sends out a sampling instruction trigger. Each amplifier 6.6', 6" in this case
output waveform and start trigger pulse generator 14, sampling trigger waveform shaping circuit 15, AND circuit IC
The output waveforms of Figure 5 (a), (b), (
C), (d), (e), and (f). Since a light source with good coherence is used for the output of the sampling trigger waveform shaping circuit 15, --like pulses can be obtained for every half wavelength of the sampling trigger light beam 1# over the entire area.

The output of the start trigger pulse generator 14 is a luminous flux 1' a
, 1'b, a step pulse is generated only when the optical path difference between them becomes 0. A start trigger generator 14 and a sampling trigger waveform shaping circuit 15 are generated by an AND circuit 16.
External trigger type A/D only when the output is on (“1”)
Converter 7' samples the data.

If the stepped mirrors 4, 4 are mechanically integrated and driven by the movable mirror driving body 9, the stepped mirrors 4, 4 can be driven by the movable mirror driving body 9, even if there is backlash in the movable mirror driving body 9, as in the above embodiment. Since the relative positions of the surfaces of the mirrors 4 and 4 do not change, accurate sampling can be performed based on the outputs of the start trigger pulse generator 14 and the sampling trigger waveform shaping circuit 15 for each scan, and jitter-free interface can be achieved. You can get a ferrogram. Therefore, noise can be reduced by simple addition and integration processing on the time axis. If the movable mirror 4'' is divided into two parts and only the stepped mirror T is configured to be able to move slightly in the direction of the beam axis, the optical path difference 2δ can be varied and the sampling window can be changed.Also, after the NO circuit 1G By inserting a frequency divider circuit, the sampling density can be changed.

(Effects of the Invention) As explained above, the optical spectrum analyzer of the present invention provides a separate interference path for the measurement start trigger even if there is drive backlash in the optical interferometer. By mechanically integrating the movable mirror, it is possible to measure the interferogram at a constant timing relative to the start trigger, so this interferogram can be simply added and integrated every time the movable mirror scans. This has the advantage that processing can be performed without jitter, and accurate measurement of extremely weak spectra can be performed.

[Brief explanation of the drawing]

Figure 1 is a block diagram of an embodiment of the present invention, Figures 2 (a) and (b) are diagrams showing interferograms of white light and a semiconductor laser, and Figures 3 (a) and (b) are additions. Figures showing spectra before and after averaging processing; Figure 4 is a configuration diagram of the second embodiment of the present invention; Figure 5(a)
6) are diagrams showing the electrical waveforms of the output of each part (each amplifier, start trigger pulse generator, sampling trigger waveform shaping circuit, and AND circuit) of the optical spectrum analyzer of the present invention, and Figure 6 is a diagram showing the electrical waveform of the output of the optical spectrum analyzer of the present invention. Figure 7 is a diagram showing the interferogram, Figure 8 is a diagram showing the calculation results of the spectrum by the fast Fourier transform device, and Figures 9 (a) and (b) are the mechanism of a conventional optical spectrum analyzer. 1... Luminous flux to be measured 1'... Luminous flux for start trigger 1'... Luminous flux for sampling trigger la, 1b+1' al 1 ' b+ 1" al
1"b -...Light flux 2.2'...Half mirror 3,3'...Fixed mirror 4.4"...Movable mirror 4'...Double-sided movable mirror 1, 7'...Step Mirror with "5.5', 5" ... light receiver (photoelectric converter) 6,
6', 6''... Amplifier 7... A/D converter 7'... External trigger type A/D converter 8... Luminous flux Fourier transform device 9... Movable mirror driver 10...・Start trigger white light source 11...Lens 12...Optical fiber 13...Collimator 14...Start trigger pulse generator 15...Sampling trigger waveform shaping circuit 16...-AND circuit Patent applicant Patent attorney representing Nippon Telegraph and Telephone Corporation
Akatsuki Sugimura Patent Attorney Sugi
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Claims (1)

[Claims] 1. The movable mirror parts of two optical interferometers are mechanically integrated with each other, the light to be measured is made incident on the first optical interferometer, and the second interferometer is provided with a wide beam such as a halogen lamp. Light from a spectral light source is incident, the interferogram signal obtained by the second interferometer is used as a start trigger signal, the interferogram of the first interferometer is measured, and Fourier transform is performed to obtain the measured light. An optical spectrum analyzer characterized by determining the optical spectrum. 2. A step is provided on the fixed mirror or movable mirror of one optical interferometer to form two interference paths, a first and a second interference path with different optical path lengths, and the light to be measured is incident on the first interference path. and input light from a wide spectrum light source such as a halogen lamp into the second interference path, and using the interferogram signal obtained in the second interference path as a start trigger signal, start the first interference path. An optical spectrum analyzer that measures the interferogram of the light and performs Fourier transformation to obtain the optical spectrum of the light to be measured. 3. A narrow spectrum light such as a helium neon laser is incident on one interferometer or interference system path, and the interferogram signal obtained from this is used as a trigger signal for measurement after generation of the start trigger signal. An optical spectrum analyzer according to claim 1 or 2. 4. The measured interferograms are added on the time axis (on the movement axis) for each movement cycle of the movable mirror and subjected to integration processing or averaging processing. The optical spectrum analyzer according to any one of the items.