JP4331387B2 - Signal discrimination method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a frequency measurement method used in the fields of optical communication and optical measurement, and more particularly to a frequency measurement method for measuring an oscillation frequency of multi-wavelength laser light with high accuracy.
[0002]
[Prior art]
In optical communication, wavelength division multiplexing (WDM) in which a plurality of signal lights having different frequencies are superimposed and transmitted to a single optical fiber has been developed. The carrier frequency of each channel is recommended by (ITU: International Telecommunication Unit), which is arranged at a position separated by an integral multiple of 100 GHz from 193.1 THz as a reference frequency. Further, the frequency accuracy of the carrier frequency is required to be 1 GHz or less. Each carrier frequency is set to satisfy the above conditions, but there is a risk of deviation from the set value due to deterioration of the light source, changes in ambient conditions, etc. Therefore, it is necessary to constantly monitor all carrier frequencies. is there.
[0003]
As a method for measuring all carrier frequencies at high speed, a method using an optical resonator having a swept resonant frequency has been proposed. FIG. 5 shows the configuration of this method. The transmission spectrum of an optical resonator has a shape in which sharp Lorentzian transmission regions are arranged at equal intervals, and the transmission peak frequency (resonance frequency) and interval (FSR: free spectrum range) are the length of the resonator and the inside of the resonator. Determined by the refractive index. When laser light is incident on an optical resonator and the amount of transmitted light is observed, if the resonance frequency is swept within the free spectral range, the emitted light is observed when the resonance frequency and the laser light frequency coincide. When laser beams from a plurality of light sources are multiplexed and incident, outgoing light is observed depending on the frequency of each laser beam. Therefore, the difference frequency between the respective light sources can be accurately estimated from the respective emitted light and the resonance frequency at that time.
[0004]
The case where there are two light sources will be described below.
It is assumed that the laser beams from the two laser light sources A and B are combined and then incident on an optical resonator whose resonance frequency is swept in a sawtooth shape. The sweep range is assumed to be sufficiently wider than the free spectrum range of the optical resonator. In this state, the amount of light transmitted through the optical resonator is observed with a light receiver. The relationship between the resonance frequency (R _i ) of the resonance mode of the optical resonator and the oscillation frequencies (νA, νB) of the light sources A and B is in the state shown in FIG. 6 and the resonance frequency is swept to the low frequency side. To do. In this case, pulse light is emitted from the optical resonator when R ₀ and R ₁ coincide with ν A by sweeping. Similarly, pulse light is emitted when R _n and R _{n + 1} coincide with νB. R _n indicates the resonance frequency of the nth resonance mode on the high frequency side with respect to R ₀ . If the resonance frequency of the optical resonator can be controlled in proportion to the output voltage of the sweep signal generator, the horizontal axis represents the output voltage of the sweep signal generator, and the vertical axis represents the light reception signal output from the light receiver. As shown in FIG. 7, periodic pulses can be observed. A1, A2, and A3 are signals from the laser light from the light source A. Since the sweep width of the resonance frequency is sufficiently wider than the free spectrum range (FSR), a plurality of signals are observed within one sweep. Similarly, B1, B2, and B3 are signals by the laser light from the light source B. If the output voltage of the signal generator at the time when the pulses A1, A2, B1, and B2 are emitted is V _A1 , V _A2 , V _B1 , and V _B2 , νA and νB are
ν _B −ν A = [n + (V _B1 −V _A1 ) / (V _A2 −V _A1 )] · FSR (1)
Can be expressed as Therefore, if the number of resonance modes n between νA and νB is known, the difference frequency between the light sources A and B can be easily measured by measuring V _A1 , V _A2 , and V _B1. It is. If n _A is known, the absolute frequency of the light source B can be measured from the equation (1).
Although the case where there are two light sources has been described above, each frequency can be obtained by the same method even when there are three or more light sources.
[0005]
[Problems to be solved by the invention]
However, in the above method, since it was previously known that the laser light amount of the light source B is stronger than the light source A, A1, A2, and A3 are from the light source A, and B1, B2, and B3 are from the light source B. If it is known as a signal but the intensity is not known, it is difficult to determine which is the signal from the light source A. In addition, when signals from the respective light sources overlap, it is very difficult to separate the respective signals.
An object of the present invention is to provide a method for discriminating which component of a received light signal is caused by the laser light of which frequency measurement light source in view of the conventional problems as described above.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, the signal discrimination method of the present invention is characterized by modulating the frequency, phase, or intensity of the frequency-measured laser beam, thereby characterizing the received signal by each frequency-measured laser beam, It is characterized in that each received light signal can be discriminated. Due to this feature, even when the received light signals overlap, the respective signals can be separated.
[0007]
That is, in the signal discrimination method of the present invention, the combined light obtained by combining the measured light having the first frequency and the measured light having the second frequency is incident on the optical resonator capable of sweeping the resonance mode, Sweep positions (A1, A2, A3) at which the measured light having the first frequency resonates when the transmitted light from the optical resonator is detected by a light receiver and the resonance mode is swept, and the second When obtaining the sweep position (B1, B2, B3) at which the measured light having a frequency resonates, the transmitted light is either the transmitted light of the measured light having the first frequency or the measured frequency having the second frequency. A signal discrimination method for discriminating whether the measurement light is transmitted light, wherein the frequency, phase, and phase of at least one of the measurement light having the first frequency and the measurement light having the second frequency To modulate at least one of the intensities There.
In the signal discrimination method of the present invention, it is possible to determine which received light signal is due to the laser light of which frequency measurement light source, and further separate the received light signals that have overlapped and the received light signals that are close to each other, so that each frequency measured laser light is separated. Can be measured with high accuracy.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of a signal discrimination method according to the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a configuration for performing a signal discrimination method according to the present invention. The laser beams from the light source A1 and the light source B2 were combined by the combiner 4 and entered into the optical resonator 3. The optical resonator 3 swept the resonance frequency by applying a voltage to the PZT with a built-in optical resonator. The laser beam emitted from the optical resonator 3 was received by the light receiver 5 and converted into a light reception signal. The received light signal was input to the analyzer 6 where the difference frequency between the two light sources was estimated. In this embodiment, the resonance frequency of the optical resonator 3 is swept by inputting the sweep signal output from the analyzer 6 to the PZT with a built-in optical resonator.
[0009]
The frequency of the light source A1 was modulated with a modulation amplitude of 400 MHz and a modulation frequency of 100 kHz. The state of the received light signal during modulation is shown in FIG. The received light signals A1, A2 and A3, which were only signals having one peak in the conventional method, were observed spreading in a crown shape. Since it was the light source A1 that added the modulation, it was possible to clearly discriminate that the signals A1, A2 and A3 spreading in a crown shape were signals from the light source A1. The center of the crown signal is the signal position of the light source A1, that is, V _A1 , V _A2 , V _A3 .
[0010]
Next, a method for obtaining the difference frequency between the two light sources from the obtained light reception signal will be described. As can be seen from FIG. 2, the peak interval becomes narrower as the sweep signal voltage becomes higher, that is, as PZT increases. This is because the expansion / contraction distance of PZT is non-linear with respect to the applied voltage. Therefore, FIG. 3 shows the result of calculating and correcting the non-linearity of PZT from FIG. Moreover, since the peak interval corresponds to the free spectrum range, the horizontal axis was converted to frequency.
[0011]
The detection of the center of the signal spread in a crown shape by modulation was performed automatically by the computer. The procedure is shown in FIG. First, the signal of FIG. 3 is peak searched to detect the signal position of the light source B2. The signal value at the peak position is set to 0, and the signal from the light source B2 is removed. The correlation with the crown signal calculated with a modulation amplitude of 400 MHz is calculated. The peak position of the correlation signal is detected, and that position is set as the signal position of the light source A1.
Through this work, the light reception signals of the light source A1 and the light source B2 were discriminated, and the respective signal positions could be accurately detected. It was possible to estimate the frequency difference between the light source A1 and the light source B2 by substituting the obtained signal position into the equation (1).
[0012]
【The invention's effect】
In the signal discrimination method according to the present invention, laser light from a plurality of frequency measurement light sources is incident on an optical resonator whose resonance frequency is swept by a sweep signal, and the laser light transmitted through the optical resonator is received by a light receiver. In the system that detects the oscillation frequency of the frequency measurement light source from the relationship between the received light signal output from the light receiver and the sweep signal, which light received signal is detected by modulating the frequency, phase, or intensity of the laser light. It was possible to clearly discern whether it was due to the frequency measurement laser beam.
Therefore, by using the signal discrimination method according to the present invention for the WDM system, the oscillation frequency of the WDM light source can be measured with high accuracy and high resolution.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a measurement system for performing a signal discrimination method of the present invention.
FIG. 2 is a diagram illustrating a relationship between a light reception signal and a sweep signal.
FIG. 3 is a diagram illustrating a relationship between a frequency and a received light signal after correcting non-linearity of PZT.
FIG. 4 is a diagram showing a procedure for obtaining the center of a crown signal.
FIG. 5 is a diagram for explaining frequency measurement using an optical resonator.
FIG. 6 is a diagram showing a relationship between a frequency of a frequency measurement laser and a resonance mode during sweeping in a frequency measurement method using an optical resonator.
FIG. 7 is a diagram illustrating a relationship between a conventional light reception signal and a sweep signal.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Frequency reference light source 2 Frequency measured light source 3 Optical resonator 4 Combiner 5 Light receiver 6 Analyzer

Claims (1)

A combined light obtained by combining the light to be measured having the first frequency and the light to be measured having the second frequency is incident on the optical resonator capable of sweeping the resonance mode, and the transmitted light from the optical resonator is received. Sweep positions (A1, A2, A3) at which the measured light having the first frequency resonates and the measured light having the second frequency resonate when the resonance mode is swept by the detector A signal for determining whether the transmitted light is transmitted light of the measured light having the first frequency or transmitted light of the measured light having the second frequency when determining the position (B1, B2, B3). A discrimination method,
By modulating at least one of the frequency, phase and intensity of at least one of the measured light having the first frequency and the measured light having the second frequency, the transmitted light is A signal discrimination method for discriminating between transmitted light of measured light having a first frequency and transmitted light of measured light having the second frequency.

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