JP4256055B2 - Frequency measurement 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. 7 shows the configuration of this method. This method sweeps the resonance frequency of an optical resonator having a very wide free spectral range of several THz, and enters a multiplexed laser beam whose frequency is to be measured, and transmits the amount of light transmitted through the optical resonator. Is used to measure the frequency of the laser beam. As shown in FIG. 8, if the lowest laser light frequency of the multiplexed laser light is ν1, and the highest laser light frequency is νM, the free spectrum range and the sweep range are νM − More than ν1 was required. As a result, only the laser beam that passed through one resonance mode could be observed. Therefore, if the horizontal axis is the frequency of a specific resonance mode during sweeping and the vertical axis is the amount of light transmitted through the optical resonator, the frequency and the amount of light of each laser beam can be measured.
[0004]
[Problems to be solved by the invention]
However, the method using an optical resonator that sweeps the resonance frequency described above has a very wide free spectral range of the optical resonator (several THz), so that the transmission width of the resonance mode is wide (several tens of GHz) and the resolution is high. There was a problem of being low.
An object of the present invention is to provide a wavelength measurement method with high resolution and high accuracy in view of the conventional problems as described above.
[0005]
[Means for Solving the Problems]
In order to solve the above-described problems, the frequency measurement method of the present invention is characterized in that by using an optical resonator with a narrow free spectrum range, the width of the resonance mode is narrowed and the resolution and accuracy of frequency measurement are improved. Yes. By setting the free spectrum range to be equal to or slightly wider than the set frequency accuracy of the laser beam to be measured, the mode of the resonance mode that transmits the laser beam can be calculated from the set frequency and the free spectrum range. It is said.
[0006]
In other words, the frequency measurement method of the present invention has an optical resonator whose free spectral range is known and whose resonance mode can be swept, and has a first frequency whose frequency is known within the range of the frequency accuracy. A frequency for measuring a difference frequency between the light to be measured having the first frequency and the light to be measured having the second frequency by combining the light and the light to be measured having the second frequency. A measurement method, the step of obtaining the number n of the resonance modes between the first frequency and the second frequency based on the first frequency, the second frequency, and the free spectrum range; The sweep position (VA1, VA2) where the measured light having the first frequency resonates when the resonance mode is swept, and the sweep position (VB1, VB2) where the measured light having the second frequency resonates. And And calculating the difference frequency based on the resonance position of the light to be measured having the first frequency, the resonance position of the light to be measured having the second frequency, the number n, and the free spectrum range. It is made up of.
The frequency measurement method of the present invention can measure the frequency of laser light with high resolution and high accuracy.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of a frequency measurement method according to the present invention will be described below in detail with reference to the drawings.
The principle of frequency measurement according to the present invention is shown below. 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 spectral range) are the resonator length and the resonator interior Determined by the refractive index. If laser light is incident on an optical resonator that has swept the resonance frequency, the emitted light is observed when the resonance frequency matches the laser light frequency. 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.
[0008]
The case where there are two light sources will be specifically 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. 1, 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 voltage of the signal generator, the output voltage of the signal generator is plotted on the horizontal axis and the amount of light transmitted through the optical resonator is plotted on the vertical axis. As can be seen, periodic pulses can be observed. P _Ai is pulsed light 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 pulses are emitted within one sweep. Similarly, P _Bi is pulsed light by laser light from the light source B. If the output voltage of the signal generator when the pulses of P _A1 , P _A2 , P _B1 and P _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 νA is known, the absolute frequency of the light source B can be measured from the equation (1).
[0009]
FIG. 3 shows a measurement system to which the frequency measurement method according to the present invention is applied. Here, two light sources are used: a frequency reference light source whose frequency is stabilized and a frequency measurement light source whose frequency is to be measured. The frequency reference light source 1 has an oscillation frequency stabilized with an absorption line of acetylene gas at 1545.1754 nm as a frequency reference. The frequency measurement light source 2 is an LD light source having an external resonator structure in which a semiconductor laser (LD), a grating, and a mirror are arranged in a Littman type. The oscillation frequency is varied by coarsely adjusting the mirror angle with a motor and finely adjusting with PZT. The frequency can be controlled with an accuracy of ± 0.6 GHz. The optical resonator 3 is a confocal etalon having a free spectral range of 1.49928 GHz and a finesse of 150. The transmittance at the resonance frequency was 20%. By applying a voltage to the built-in PZT, the resonance frequency can be swept up to about 3 times the free spectrum range.
[0010]
Since the frequency accuracy of the frequency measurement light source is 0.6 GHz while the free spectral range of the optical resonator is about 1.5 GHz, n in Equation (1) can be calculated without miscounting.
Laser beams from the frequency reference light source 1 and the frequency measurement light source 2 were combined by a multiplexer 4 and incident on an optical resonator 3. The optical resonator 3 sweeps 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. FIG. 4 shows a graph in which the light reception signal voltage is plotted on the vertical axis and the sweep signal voltage is plotted on the horizontal axis. In the figure, T1, T2, and T3 are signals from the frequency-measurement light source 2, and R1, R2, and R3 are signals from the frequency reference light source 1. In order to clearly distinguish the signals of the frequency reference light source 1 and the frequency measured light source 2, the output of the frequency reference light source 1 was reduced by 5 dB compared with the frequency measured light source 2 and input to the optical resonator 3. As can be seen from FIG. 4, as the sweep voltage increases, that is, as the PZT increases, the peak interval becomes narrower. This is because the expansion / contraction distance of PZT is non-linear with respect to the applied voltage. Therefore, FIG. 5 shows the result of calculating and correcting the nonlinearity of PZT from FIG. Moreover, since the peak interval corresponds to the free spectrum range, the horizontal axis was converted to frequency.
[0011]
Peak position detection is automatically performed by a computer, and n calculated from the signal position, the oscillation frequency of the frequency reference light source 1, the set frequency of the frequency measurement light source 2 and the oscillation frequency of the frequency reference light source 1 is substituted into equation (1). Thus, the oscillation frequency of the frequency measurement light source 2 was estimated.
[0012]
The wavelength dependence of the measurement accuracy of the oscillation frequency of the frequency measurement light source 2 measured by the method of the present invention was measured with reference to the measurement value of a high-accuracy wavelength meter (resolution: 10 MHz). The results are shown in FIG. The frequency range that can be oscillated by the frequency measurement light source used this time was 1480-1555 nm, so measurements were only made within this wavelength range. It can be seen from the figure that the frequency can be measured with an accuracy of ± 0.1 GHz even in a wavelength region 65 nm away from the frequency reference light source. Some observations have shown that the measurement accuracy has deteriorated to 0.2 GHz or more, but this is because the frequency measurement light source has oscillated in multiple modes.
In this embodiment, there are two light sources. However, even if the number of light sources is increased to three or more, the respective laser light frequencies can be measured in the same manner.
[0013]
【The invention's effect】
In the frequency measurement 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 light reception signal output from the light receiver and the sweep signal, the fact that the frequency of the laser light is known with certain accuracy in advance is used. Even if the free spectrum range of the resonator is narrow, the frequency of the frequency measurement light source can be measured with high accuracy. Since the free spectrum range is narrow, the resonance mode width can also be narrowed, resulting in high resolution and high accuracy.
Therefore, by using the frequency measurement 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 for explaining a frequency measurement method of the present invention, and is a diagram showing a relationship between a frequency of light to be measured and a resonance frequency of an optical resonator.
FIG. 2 is a diagram for explaining a frequency measurement method according to the present invention, and is a diagram showing a relationship between a transmitted light amount of an optical resonator and a sweep signal.
FIG. 3 is a diagram showing a measurement system to which the frequency measurement method of the present invention is applied.
FIG. 4 is a diagram illustrating a relationship between a sweep signal and a light reception signal.
FIG. 5 is a diagram illustrating a relationship between a frequency and a light reception signal after correcting the nonlinearity of PZT.
FIG. 6 is a diagram showing measurement results obtained by the frequency measurement method of the present invention.
FIG. 7 is a diagram for explaining frequency measurement using an optical resonator.
FIG. 8 is a diagram showing the relationship between the frequency of light to be measured and the resonance mode during sweep in the conventional frequency measurement method.
[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)

An optical resonator whose free spectral range is known and whose resonance mode can be swept is measured light having a first frequency and a second frequency whose frequency is known within the range of the frequency accuracy. A frequency measurement method for measuring a difference frequency between a light to be measured having the first frequency and a light to be measured having the second frequency by entering a combined light that is combined with light,
Determining the number n of the resonance modes between the first frequency and the second frequency based on the first frequency, the second frequency, and the free spectral range;
Sweep positions (VA1, VA2) where the measured light having the first frequency resonates when the resonance mode is swept, and sweep positions (VB1, VB2) where the measured light having the second frequency resonates. The stage of seeking
Measuring the difference frequency based on the resonance position of the light to be measured having the first frequency, the resonance position of the light to be measured having the second frequency, the number n, and the free spectrum range. Method.

Images