JP2664255B2 - Optical spectrum analyzer - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical spectrum analyzer for measuring a spectrum of an LD or the like used in a coherent optical communication system.

(Prior Art) In the coherent optical communication system, it is necessary to measure the line width and spectrum of an LD with very high resolution. As a high-resolution optical spectrum analyzer, one using a Fabry-Perot resonator, one using a delayed self-method, and the like are known. However, the former has the disadvantage that it does not provide sufficient resolution to measure LDs for coherent optical communication, and the latter provides sufficient resolution but cannot separate the spectral components when multiple spectra exist. There is. Therefore, recently, a method of analyzing an optical spectrum using a method called a heterodyne method or a homodyne method has been considered.

Conventionally, a practical apparatus of an optical spectrum analyzer has a configuration as shown in FIG. 5 and mainly uses a diffraction grating (grating).

That is, in FIG. 5, the measured light input from the slit 51 is converted into a parallel light by the
It is incident on 53. The light to be measured having a wavelength λ determined by the inclination angle with respect to the incident light of the diffraction grating (grating) 53 reflected by the diffraction grating 53 is reflected by a collimator 54,
The light is input to the light receiver 56 through the slit 55. The light receiver 56 outputs a light intensity signal proportional to the light intensity of the incident light. The inclination angle of the diffraction grating 53 with respect to the incident light is controlled by a drive mechanism 57 including, for example, a motor.

In such an optical spectrum analyzer, when the diffraction grating 53 is rotated by the driving mechanism 57, the above-described tilt angle changes sequentially, and the wavelength λ included in the light to be measured reflected by the diffraction grating 53 is continuously changed. Changes to Therefore, when the relationship between each signal level of the light intensity signal output from the light receiver 56 with respect to each rotation angle of the diffraction grating 53 is graphed, the spectral characteristics of the measured light can be obtained.

However, this method has a drawback that it takes a long time for sweeping speed and analysis as an optical spectrum measuring instrument. Further, since the device itself uses a grating, its shape becomes large, which is an obstacle to miniaturization.

Conventionally, there are the following optical spectrum analyzers based on the heterodyne method or the homodyne method. In FIG. 4, first, local light from a light source 43 called a local light source is multiplexed with a light to be measured by a multiplexer 41. When this combined output light is detected by the light receiver 45, it has a frequency component of the difference between the oscillation frequency of the measured light and the oscillation frequency of the local light,
An electric signal proportional to the product of the respective optical electric field strengths is obtained. The oscillation frequency of each light is extremely high (several 100T
Hz), when the wavelengths (frequency) of the two lights are close, the difference frequency (intermediate frequency) becomes a frequency that can be directly detected. The heterodyne method measures this signal while limiting it to a desired frequency range with a band-pass filter. The two light wavelengths are equal, and the frequency to be detected is a low frequency, that is,
The homodyne method is a method in which the frequency range is limited by using a low-pass filter near 0 Hz for measurement. Now, when the light to be measured has a plurality of spectra, if the oscillation wavelength (frequency) of the local light is changed, the level detected through the detector 47 after the filter 46 will sequentially change the spectrum of the light to be measured. I will follow. This is the principle of an optical spectrum analyzer using a heterodyne type (homodyne type). In FIG. 4, reference numeral 40 denotes a sweep signal generator, and 49 denotes a display.

(Problems to be Solved by the Invention) However, such an optical spectrum analyzer based on the homodyne method or the heterodyne method has the following disadvantages.

In the homodyne method, since the measurement levels are significantly different due to the phase relationship at the time of multiplexing of the local light and the measured light, there is no problem when the light levels of the local light and the measured light are equal and the phases match. In the case of phase, there may be no signal at all.

In the heterodyne method, an electric circuit of AC coupling is usually used, and in this case, an intermediate frequency of about several MHz to several GHz is selected. Further, the frequency to be measured and the local frequency are operated apart from each other, and the frequency is used within a range that does not change the magnitude relation of the frequencies. This method uses a light source to be measured that has a high-purity spectrum in the case of optical communication.
The frequency width to be used is narrow and poses no practical problem. However, in the case of an optical spectrum analyzer, it is necessary to assume a case where the measured light has a wide frequency width having a plurality of spectra. In this case, the area that cannot be measured by the above method is large. For this reason, the local light source must have a variable wavelength (frequency) along the wavelength range of the light source to be measured. However, in this case, a signal is present in both cases where the frequency difference between the local light and the measured light is + and-, and accurate measurement cannot be performed.

Specifically, according to the conventional heterodyne method,
Produces a beat between the local wavelength λ _L and the input wavelength λ,
A wave of λ−λ _L is detected from the output of the multiplexer, but | ±
A fatal drawback occurs for a spectrum analyzer in which two separated waveform images of (λ−λ _L ) | appear on the display screen twice apart with time (with the elapse of the sweeping time). Was.

 FIG. 3A illustrates this state.

With respect to the input light λ, signals of | λ−λ ₁₁ | and signals of wavelengths corresponding to | λ−λ ₁₂ | appear at the output of the bandpass filter with the local lights λ ₁₁ and λ ₁₂ . As a result, signals appear as if different signals were received in accordance with the sweep of the local light on the display screen.

Therefore, it cannot be used as a broadband optical spectrum analyzer using a conventional IF wave as shown in Prior Art 1 (Japanese Patent Laid-Open No. 1-177520). It is an object of the present invention to address this point.

(Means and Actions for Solving the Problems) In order to solve the above problems, according to the present invention, a local light output unit, and a sweep signal generation unit for sweeping the oscillation wavelength of local light output from the local light output unit A multiplexer for multiplexing the local light and the measured light; a photodetector for outputting an electric detection signal of the multiplexed light; and a center frequency having a frequency substantially equal to the spectrum line width of the local light. A band-pass filter that filters the electric detection signal; a detector that detects a signal output by the band-pass filter; and a receiver that receives an output signal of the detector and a sweep signal of the sweep signal generator. And a display for displaying the output signal of the detector corresponding to the wavelength, thereby providing a heterodyne optical spectrum analyzer.

That is, the gist of the present invention is to focus on the fact that the spectrum spread (spectrum line width) of the local light is usually equal to or more than a predetermined width l,
A local light multiplexes are wavelength sweep, when the wavelength lambda _L of the local light becomes very near the wavelength lambda of the light to be measured, so as to obtain an intermediate frequency signal of a low F _I component from the combined light And then convert the combined light into an electrical signal
Lies in substantially have like the spectrum line width of the local light that are to be decided analyzable resolution center frequency of the band-pass filter for obtaining the intermediate frequency signal F _I component as an optical spectrum analyzer.

Accordingly, the display image of the wavelength spectrum on the logical display is, in other words, before and after the wavelength λ _{L of the} local light is swept and crosses the wavelength λ of the light to be measured (λ−λ _L , λ _L −λ). For example, it appears two positions very close to the position corresponding to the input wavelength λ of the measured light.

However, as described above, in reality, there is a spread l of the spectrum of the combined light of the local light and the light to be measured, so that the display image appears to have a single width.

The width when these two display images appear as one apparently becomes steeper in display as the center frequency of the bandpass filter becomes lower.

Therefore, in the present invention, a band-pass filter having a center frequency equal to the spectrum line width (limit of resolution) of the local light, which is the effective limit, is used.

As a result, the above-described two display images (signal waveforms)
Is observed as one waveform on the display, and the center frequency of the waveform coincides with the center frequency of the measured light.

 FIG. 3B shows the state of the display screen.

1) That is, the line width of the local light is made sufficiently narrower than the line width of the light to be measured. 2) When the frequency of the band-pass filter is made substantially equal to the line width of the local light. The configuration is sufficiently narrower than the line width.

As shown in the figure, the line width of the measured light is β, the line width of the local light is 1, the frequency of the band-pass filter is F _I , the frequency of the measured light is λ, the frequency of the local light is λ _L , and the width of the band-pass filter. Is ΔF. In this case, the detected signal is λ _L −λ
Or has a frequency component of λ−λ _L and a width of l,
It becomes a signal determined by β and ΔF. The light to be measured is 1),
When the relationship of 2) is satisfied, the level of the optical signal to be measured can be sufficiently measured, and the spectrum of the optical signal in a wide range can be measured by moving λ _L in a wide range.

Embodiment An embodiment of the present invention will be described below in detail with reference to the drawings.

In the present invention, as shown in FIG. 1 (a), in order to configure a heterodyne optical spectrum analyzer for displaying a wavelength component of input light in a broadband spectrum,
Intermediate frequency processing unit for processing the generated intermediate frequency signal lambda _I
5, 6, and 7 are designed to pass only relatively low wavelength components, and have a band filter (BPF) for that purpose.

Further, a device has been devised to remove fluctuation noise due to deflection, which is a phenomenon unique to optical signals.

That is, as shown in FIG. 1 (a), for an unknown input light λ having a wide band,
_A variable local light generator 3 which continuously changes _L and outputs (outputs a local wave); a multiplexer 1 which inputs and combines and outputs both the measured light and the local light; and A photodetector 5 for receiving the output of the multiplexer 1 and performing photoelectric conversion, a band-pass filter (hereinafter referred to as BPF) 6 for passing the converted electric signal by a predetermined frequency width, and a detector 7 for detecting the electric signal; A display 9 for displaying the detected signal, and a sweep signal generator 10 for generating a sweep signal for sweeping the display 3 in synchronization with the sweep of the variable local light generator 3 and the sweep.

Thereby, as shown in FIG. 2, the width of the wavelength is λ and the corresponding spectrum level is displayed on the display 9 in the vertical direction, and the optical spectrum can be observed.

A light deflection surface disturbance means 2b for disturbing the local light deflection surface between the variable local light generator 3 and the multiplexer 1;
And rotating or disturbing the light deflecting surface at a speed that is at least one order of magnitude faster than the time required to sweep one of the spectral line widths of the desired spectrum signal of the sweep signal.

By doing so, the effect of canceling the light due to the phase difference between the input light and the local light is reduced, and an image with a substantially averaged display screen can be displayed. FIG. 1 (b) shows the change of the display screen due to the disturbance of the deflection surface (a → b →
The state of c) is shown.

The means for disturbing or scattering the light deflecting surface can be obtained by flowing an optical signal through an optical fiber and applying high-speed mechanical or electromagnetic vibration to the fiber. Such a disturbance can also be obtained by rotating a λ / 4 deflection plate at a high speed and passing light therethrough.

Even if the above-mentioned cancellation effect is obtained, as long as the method of presenting an image of two adjacent spectra according to the present invention, one of the two remains stochastically, and the image disappears from the position where it should appear. There is an advantage that the drawback of decay and attenuation is doubled.

Further, the line width of the local light is at least 1/2 or less of the spectrum line width of the measured light, and both the set frequency and the bandwidth of the filter are at least T / 2 (β) of the spectrum line β of the measured light. / 2) If the width is less than or equal to the width, a spectrum analyzer with good resolution can be configured.

As the variable local light generator 3, besides the one that directly changes the light wavelength using an LD (laser diode) and outputs the same, the variable local light generator that applies SAW or radiates ultrasonic waves to a light propagation medium to generate coherent light An ultrasonic light modulation technique for partially changing the density and stress in a medium through which light passes may be used. In this case, a plurality of the light generators may be used in cascade.

(Effects of the Invention) As described above, according to the present invention, the spectrum of the light to be measured in a wide wavelength (frequency) range can be obtained without instability unlike the conventional homodyne method, and with the local frequency minus the heterodyne method. A good measurement can be performed without any misunderstanding of the measured value due to combining and displaying both the positive and negative (±) cases of the measured frequency.

 That is, specifically, the following effects are obtained.

1) Since the image is displayed excluding the λ _O (center portion of λ) portion having a large variation ratio, the image of the spectrum signal on the display screen becomes stable.

2) Even if the light source has an unstable light deflecting surface, a practical device of a broadband optical spectrum analyzer can be configured relatively unaffected by the light source.

3) A compact and lightweight optical spectrum analyzer can be realized with a simple configuration.

4) An effective and effective optical spectrum analyzer utilizing the advantages of the heterodyne system can be realized.

[Brief description of the drawings]

FIG. 1 (a) is a block diagram showing the present invention, and FIG. 1 (b)
FIG. 2 is a diagram showing a waveform of the present invention, FIG. 2 is a schematic diagram showing a display waveform of the present invention, FIG. 3 (a) is a diagram showing the principle of the heterodyne method, and FIG. 3 (b) is a display screen of the present invention FIG.
4 and 5 are block diagrams showing different conventional examples. In the figure, 1 ... combiner, 2b ... means for disturbing light polarization plane, 3 ...
Variable local light generator, 5 ... photodetector, 6 ... BPF,
7 Detector, 9 Display, 10 Sweep signal generator,
A, B, C: IF indicating the state of change of the display screen due to polarization disturbance, IF: Intermediate frequency signal, λ: wavelength of light to be measured, λ
_{L ......} local light, β ...... measured light line width, l ...... local light line width, the passband band ΔF ...... BPF, the pass frequency of F _I ...... BPF.

Claims (1)

(57) [Claims] A local light output unit; a sweep signal generation unit for sweeping an oscillation wavelength of the local light output from the local light output unit; a multiplexer for multiplexing the local light and the light to be measured; A photodetector that outputs an electric detection signal of the multiplexed light, a band-pass filter that has a frequency substantially equal to the spectrum line width of the local light at a center frequency, and filters the electric detection signal, A detector for detecting a signal output by the pass filter, and a display for receiving the output signal of the detector and the sweep signal of the sweep signal generator and displaying the output signal of the detector in correspondence with the wavelength. Equipped heterodyne optical spectrum analyzer.