JPH10107355A - Frequency selective oscillator and oscillating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a frequency-selective oscillator, capable of deciding a plurality of reference frequencies at equal frequency intervals with high pecision as well as easily select the laser beams in one frequency out of a plurality of reference frequencies, without deteriorating the precision of the laser beams. SOLUTION: This frequency-selective oscillator 10, having an optical amplifier 12 is provided with an etalon 16 for deciding a plurality of reference frequencies comprising transmittable frequencies by transmitting laser beams in a plurality of frequencies at equal frequency intervals of 100Ghz on an optical path 14 of a ring-type resonator 10a oscillating laser beams as well as a frequency selective filter 18 for selecting are frequency out of these reference frequencies as a selective reference frequency series-connected to the etalon 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a frequency selective oscillator and a frequency selective oscillating method suitable for use in optical communication, optical measurement, and manufacturing of optical equipment, particularly, frequency-frequency multiplex communication.

[0002]

2. Description of the Related Art In optical wavelength multiplex communication, signals of a plurality of channels are transmitted using a common optical fiber. Each of these channels is assigned a plurality of frequencies at a fixed frequency interval. In optical wavelength multiplex communication, it is necessary to prevent signals of different channels from interfering with each other. To do so, the transmission system,
In the entire communication system including the reception system and the propagation system between the transmission system and the reception system, the frequency of each channel needs to be the same.

[0003] In August 1996, ITU-T (International Telecommunication Union Telecommunications Research Committee), which is an international organization, designated "Nom" as the frequency of light used for optical wavelength division multiplexing communication.
An international standard has been determined, referred to as "inal Central Frequencies". Hereinafter, for convenience, a plurality of frequencies determined by the international standard will be referred to as “standard frequencies”.

[0004] As a result of the determination of the standard frequency, it is considered that a frequency selective oscillator that can obtain light of any frequency among a plurality of standard frequencies used for optical wavelength division multiplexing as necessary is required.

[0005] By the way, conventionally, a dedicated device for such a frequency selective oscillator has not been known. Thus, as such a frequency selective oscillator, for example, a method in which a plurality of light sources each oscillating a plurality of predetermined frequencies are prepared and the oscillation frequency is switched by a switching element is considered.

[0006]

However, ordinary switching elements are expensive and cause a large loss of light. For this reason, it is considered that the configuration in which the oscillation frequency is switched by the switching element is not appropriate as the frequency selective oscillator. Further, in the case of this configuration, in order to prepare a plurality of frequencies, it is necessary to prepare many expensive light sources and adjust the wavelength.

Therefore, a plurality of standard frequencies at equal frequency intervals can be obtained with high accuracy, and light of any one of the plurality of standard frequencies can be easily selected without lowering the accuracy. It has been desired to realize an oscillator and a frequency selective oscillation method.

[0008]

[Means for Solving the Problems]

(First Invention) According to the frequency selective oscillator of the first invention according to this application, an optical amplifier is provided, and only light of a plurality of frequencies at equal frequency intervals is provided on the optical path of a resonator that oscillates laser light. An etalon for determining a plurality of standard frequencies consisting of frequencies that can be transmitted by transmission and a frequency selection filter for selecting any one of the standard frequencies are provided in series with each other. .

In the frequency selective oscillator according to the first aspect of the present invention, preferably, the resonator is a ring-type resonator.

(Second Invention) A second invention according to this application is described.
According to the frequency selective oscillation method of the invention, an etalon and a frequency selective filter are provided in series with each other on the optical path of a resonator that oscillates laser light, and the etalon allows the etalon to pass through the etalon. Determine a plurality of standard frequencies in the interval, and by the frequency selection filter,
It is characterized in that any one of the standard frequencies is selected.

Further, in the frequency selective oscillation method according to the second invention, it is preferable to use a ring type resonator as the resonator.

As described above, according to the frequency selective oscillator of the first invention and the frequency selective oscillation method of the second invention, an etalon is used to determine a plurality of standard frequencies at equal frequency intervals. The etalon has a property of transmitting only light having a frequency of discrete values at equal frequency intervals. The frequency of the light that can pass through the etalon is physically determined by the thickness of the etalon if the incident angle of the light to the etalon and the temperature conditions are constant. The accuracy of the standard frequency determined by the etalon is determined by the parallelism, the surface accuracy, and the reflectance of both sides of the etalon.

According to these inventions, light having a desired frequency is selected by the frequency selection filter from the standard frequencies determined by the etalon. As a result, even if the frequency selected by the frequency selection filter changes continuously, the frequency that can be extracted is limited to the standard frequency of discrete values determined by the etalon. For this reason, even if the frequency accuracy selected by the frequency selection filter is low, it is possible to easily oscillate by selecting a desired frequency (selected standard frequency) from the standard frequencies without lowering the accuracy.

As a result, according to these inventions, it is possible to easily oscillate light of the selected standard frequency with high accuracy by switching channels.

In these inventions, if the resonator is a ring-type resonator, the ring-type resonator may be referred to as "spatial hole burning" which may be called "spatial amplification saturation phenomenon".
Since g) does not occur, the oscillation wavelength is easily stabilized.

The accuracy of the standard frequency transmitted through the etalon is highest when the etalon is arranged perpendicular to the optical axis (when the etalon is arranged so that the laser beam is perpendicularly incident on the incident surface of the etalon). . If a ring-type resonator is used, a composite resonator is not formed, which is suitable for disposing the etalon perpendicular to the optical axis.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, with reference to the drawings, embodiments of a frequency selective oscillator according to a first invention and a frequency selective oscillation method according to a second invention according to the present application will be described together. The drawings referred to merely schematically show the sizes, shapes, and arrangements of the components so that these inventions can be understood. Therefore, these inventions are not limited to the illustrated examples.

(First Embodiment) First, a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a configuration diagram for explaining the frequency selective oscillator according to the first embodiment.

The frequency selective oscillator 10 includes an optical amplifier 1
2 for determining a plurality of standard frequencies consisting of frequencies that can be transmitted by transmitting only light of a plurality of frequencies at equal frequency intervals on the optical path 14 of the ring resonator 10a that oscillates laser light. An etalon 16 and a frequency selection filter 18 for selecting any one of the standard frequencies as a selected standard frequency are provided in series with each other.

The optical path 1 of the ring resonator 10a
A semiconductor laser amplifier (LDA) 12 is provided on the optical amplifier 4 as an optical amplifier 12. This LDA12 is
A semiconductor laser 20 and a pair of collimator lenses 22 and 24 are provided. The LDA 12 oscillates a laser beam having a wavelength of about 1500 nm.

The laser light emitted from the LDA 12 is
After being sequentially reflected by the mirror 26 and the second mirror 28, the light passes through the first isolator 30 and enters the etalon 16.

The etalon 16 is the etalon 1
6, only light of a plurality of standard frequencies at equal frequency intervals is transmitted. The standard frequency is determined by the temperature and thickness of the etalon 16. The temperature of the etalon 16 is controlled by a Peltier device 32 provided in contact with the etalon 16. The thickness of the etalon is about 2 mm, and the reflectance is 95%.

The frequency interval Δν of the standard frequency transmitted through the etalon is given by the following equation (1).

Δν = c / 2ln (1) where c is the speed of light, l is the thickness of the etalon, and n is the refractive index.

The standard frequency transmitted through the etalon 16 is as follows:
It can also be controlled by the incident angle of light on the etalon 16. However, it is desirable that the light be incident on the incident surface of the etalon 16 substantially perpendicularly. This is because the smaller the incident angle of the etalon 16 with respect to the normal to the incident surface, the smaller the light loss and the higher the accuracy.

The light of the standard frequency transmitted through the etalon 16 enters the frequency selection filter 18. In this embodiment, a transmission type filter 18 is used. This filter is
It is composed of a dielectric multilayer film. The transmission frequency of the filter 18 changes depending on the incident angle of light on the filter. Therefore, this filter 18
Selects one of a plurality of standard frequencies as a selected standard frequency by changing an incident angle. In FIG. 1, the illustration of the rotation mechanism of the filter 18 is omitted.

The light of the selected standard frequency transmitted through the filter 18 is supplied to the second isolator 34 and the half-wave plate (λ / 2
After sequentially passing through the plate 36, the light enters a beam splitter 38. The λ / 2 plate 36 and the beam splitter 3
8 constitutes a variable beam splitter. Part of the light that has entered the beam splitter 38 is reflected by the third mirror 40 and then enters the LDA 12. A part of the light incident on the beam splitter 38 exits from the ring resonator 10a and is
Then, the light enters the optical fiber 44 via the optical fiber 2. Then, oscillation light is extracted from the optical fiber 44.

Next, the principle of frequency selection will be described with reference to FIG. 2A to 2C show frequency characteristics, the horizontal axis of FIGS. 2A to 2C represents frequency (arbitrary unit), and the vertical axis represents light intensity or light transmission intensity (arbitrary unit). Unit).

FIG. 2A is a schematic diagram showing a resonator mode of the ring resonator. The interval between the resonator modes indicated by the curve I in FIG. 2A is, for example, a frequency interval of 1 GHz. This is for light with a wavelength of about 1500 nm,
This corresponds to a wavelength interval of about 0.008 nm.

FIG. 2B is a schematic diagram showing the transmission frequency characteristics of the etalon. If an etalon is used, a plurality of standard frequencies at equal frequency intervals can be obtained. Further, the frequency interval Δf of the transmission peak II of each standard frequency is
For example, about 100 GHz, and finesse is 10 to 20
The degree can be easily obtained. This is 1500
In the case of light having a wavelength of about nm, this corresponds to a wavelength interval of about 0.8 nm.

FIG. 2C is a schematic diagram showing the transmission frequency characteristics of the filter. The half width of the transmission peak III is wider than the half width of each transmission peak II of the etalon. The half-width of the transmission peak II is
Those having a thickness of about 0.5 nm to 1.0 nm can be easily obtained.

In selecting the frequency, a transmission peak P included in the transmission peak III of the filter is selected from the transmission peaks II of the etalon, and further,
One of the modes I of the resonator that matches the transmission peak P of the etalon is selected as the oscillation frequency.

The accuracy of the selected standard wavelength is first determined by the etalon. This is because the half width of the transmission peak II of the etalon is different from the transmission peak II of the filter.
In addition to being narrower than the full width at half maximum of I, the etalon cuts off light at frequencies between
This is because II has been determined. Therefore, even if the transmission frequency of the filter continuously changes, the selected standard frequency that can be extracted is limited to the standard frequency of discrete values determined by the etalon. Therefore, even if the frequency accuracy selected by the frequency selection filter is lower than the accuracy of the standard frequency transmitting the etalon, a desired frequency (selected standard frequency) can be easily selected from the standard frequencies without lowering the accuracy. be able to.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 3 is a configuration diagram for explaining a frequency selective oscillator according to the second embodiment.

In FIG. 3, the same components as those of the frequency selective oscillator of the first embodiment shown in FIG. 1 are denoted by the same reference numerals as those shown in FIG. Detailed description is omitted.

The frequency selective oscillator 50 according to the second embodiment
Is the frequency selective oscillator 10 of the first embodiment described above.
A prism 46 is provided instead of the first mirror 26 and the second mirror 28, and a polarizer 48 is provided instead of the third mirror 40. And this frequency selective oscillator 50
In, the etalon 16 and the frequency selection filter 18 are arranged on the optical path 14 of the ring resonator 50a such that light transmitted through the frequency selection filter 18 is incident on the etalon 16. That is, the order of incidence of light on the etalon and the frequency selection filter is not limited.

The frequency selective oscillator 50 according to the second embodiment
Also, with a simple configuration, similarly to the first embodiment, a plurality of standard frequencies at equal frequency intervals can be obtained with high accuracy, and any one of the plurality of standard frequencies can be obtained. Light can be easily selected without reducing accuracy.

In the first and second embodiments, since the resonator is a ring-type resonator, spatial fall burning does not occur, so that the oscillation wavelength is easily stabilized. Since the etalon is arranged perpendicular to the optical axis, the accuracy of the standard frequency transmitting the etalon can be further improved. Further, in these embodiments, by inserting an optical isolator, the oscillation frequency is further stabilized to improve the accuracy.

[0039]

Next, with reference to FIG. 4, it will be described that the accuracy of the selected standard frequency is determined not by the filter but by the etalon. FIG. 4 is a graph showing a measurement result of a relationship between a scale value of a micrometer for adjusting an incident angle with respect to a filter whose transmission wavelength changes according to the incident angle and a selected standard frequency of light output from a frequency selective oscillator. The horizontal axis of FIG. 4 represents the value of the micrometer scale,
The vertical axis represents the frequency (THz). The value of the micrometer scale corresponds to the angle of incidence of light on the filter, and the angle of incidence corresponds to the transmission frequency of the filter. Therefore, the horizontal axis of the graph of FIG. 4 corresponds to the transmission center frequency of the filter. The output intensity of the frequency selective oscillator at the time of this measurement was about 0.1 mW.

As shown in the graph of FIG. 4, even if the transmission frequency of the filter corresponding to the value of the micrometer scale is changed, if the transmission frequency of the filter is within a certain range,
The value of the selected standard frequency is constant. When the transmission frequency of the filter is changed by a certain value or more, the selected standard frequency shifts to a value separated by 50 GHz. In this way, the selected standard frequency changes stepwise.

Accordingly, it can be seen from the graph of FIG. 4 that the transmission frequency of the filter does not have to be strictly adjusted to the desired selected standard frequency. It can be seen that if the magnitude is within a certain range, a desired selected standard frequency can be easily obtained without lowering the precision. Therefore, according to the present invention, an accurate selected standard frequency can be easily obtained so as to switch channels.

Next, the measurement results of the transmission wavelength characteristic of the etalon are shown in the graph of FIG. The horizontal axis of the graph in FIG. 4 represents the wavelength (μm), and the vertical axis represents the light transmission intensity (dBm).
As shown by a curve IV in the graph of FIG. 4, transmission wavelength peaks appear at wavelength intervals corresponding to 50 GHz intervals. However, since the frequency interval is equal, the wavelength interval of the oscillation wavelength peak is not exactly equal.

Next, the graph of FIG. 6 shows, as a measurement result of the output of the frequency selective oscillator having the configuration described in the second embodiment, a measurement result obtained by oscillating a plurality of selected standard frequencies. .

In this measurement, the frequency interval of the resonator mode was 2.3 GHz, which was 1500 nm.
In the case of light having a wavelength of the order, this corresponds to a wavelength interval of about 0.0184 nm. The transmission frequency interval of the etalon in this measurement is 50 Hz, which corresponds to about 0.4 nm in the case of light having a wavelength of about 1500 nm.

The horizontal axis of the graph of FIG. 6 is the wavelength (μm).
And the vertical axis represents light intensity (dBm). The oscillation wavelength peaks a to l shown in the graph of FIG. 6 correspond to the transmission wavelength peaks a to l in the graph of FIG. 5, respectively. Here, every other transmission wavelength peak of the etalon is selected and set to 1
Light of a selected standard frequency is oscillated at intervals of 00 GHz.

In each of the above embodiments and examples,
Although these inventions have been described only for examples in which specific materials are used and specific conditions are employed, these inventions can be modified and modified in many ways. For example, in the above-described embodiment, a transmission type filter whose transmission frequency changes according to the incident angle of light is used as the frequency selection filter. However, in these inventions, various filters can be used. For example, a so-called “non-polarization type wavelength tunable filter” whose transmission frequency changes depending on the light transmission position can be used as the frequency selection filter.

In these inventions, the frequency selection filter does not need to be limited to a transmission type filter. For example, a grating can be used as the frequency selection filter. Note that, when a grating is used as a frequency selection filter, it is different from a case where a wavelength is simply selected only by the grating. That is, in the present invention, since a plurality of standard frequencies are determined by the etalon, even if the setting of the grating as the frequency selection filter is slightly shifted, the accuracy is not reduced if the size of the shift is within a certain range. A desired standard frequency can be selected from the standard frequencies.

In each of the above-described embodiments and examples, only one of the standard frequencies is selected for oscillation. In the present invention, a plurality of frequencies are selected at the same time. Is also good. In this case, for example, it is conceivable to remove the isolator provided in the ring resonator, or to oscillate a plurality of frequencies simultaneously by applying high-speed modulation to the LDA.

In each of the above embodiments and examples, an LDA is used as an optical amplifier. However, in these inventions, the optical amplifier is not limited to this, and for example, a polarization maintaining EDFA (Erbium-doped fiber).

[0050]

According to the frequency selective oscillator of the first invention and the frequency selective oscillation method of the second invention, the etalon is used to determine a plurality of standard frequencies at equal frequency intervals. Then, light of a desired frequency is selected from the standard frequencies by a frequency selection filter. As a result, even if the frequency selected by the frequency selection filter changes continuously, the frequency that can be extracted is limited to the standard frequency of discrete values determined by the etalon. Therefore, even if the frequency accuracy selected by the frequency selection filter is lower than the accuracy of the standard frequency transmitting the etalon, a desired selected standard frequency can be easily selected from the standard frequencies without lowering the accuracy. .

In these inventions, if the resonator is a ring resonator, spatial fall burning does not occur in the ring resonator, so that the oscillation wavelength is easily stabilized.

The accuracy of the standard frequency transmitted through the etalon is highest when the etalon is arranged perpendicular to the optical axis. If a ring-type resonator is used, a composite resonator is not formed, which is suitable for disposing the etalon perpendicular to the optical axis.

Therefore, according to these inventions, it is possible to accurately obtain a plurality of standard frequencies at equal frequency intervals with a simple structure, and to accurately detect light of any one of the plurality of standard frequencies. Can be easily selected without dropping.

[Brief description of the drawings]

FIG. 1 is a configuration diagram for explaining a frequency selective oscillator according to a first embodiment;

FIG. 2A is a schematic diagram illustrating a resonator mode of a ring resonator, FIG. 2B is a schematic diagram illustrating a transmission frequency characteristic of an etalon, and FIG. 2C is a schematic diagram of a frequency selective oscillator. It is a schematic diagram which shows an oscillation frequency characteristic.

FIG. 3 is a configuration diagram for explaining a frequency selective oscillator according to a second embodiment;

FIG. 4 is a graph showing a measurement result of a relationship between a scale value of a micrometer and a selected standard frequency.

FIG. 5 is a graph showing measurement results of transmission wavelength characteristics of an etalon.

FIG. 6 is a graph showing the results of measuring outputs of a plurality of standard frequencies of the frequency selective oscillator according to the second embodiment.

[Explanation of symbols]

 10, 50: frequency selective oscillator 10a, 50a: ring resonator 12: optical amplifier, semiconductor laser amplifier (LDA) 14: optical path 16: etalon 18: frequency selective filter, filter 20: semiconductor laser 22, 24: collimator lens 26 : First mirror 28: second mirror 30: first isolator 32: Peltier element 34: second isolator 36: half-wave plate (λ / 2 plate) 38: beam splitter 40: third mirror 42: collimator 44: optical fiber 46: Prism 48: Polarizer

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yoshitaka Nawahira 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. An optical amplifier, comprising: transmitting only light of a plurality of frequencies at equal frequency intervals on an optical path of a resonator that oscillates a laser beam; And a frequency selection filter for selecting any one of the standard frequencies in series with each other. 2. The frequency selective oscillator according to claim 1, wherein said resonator is a ring-type resonator. 3. An etalon and a frequency selection filter are provided in series on an optical path of a resonator that oscillates a laser beam, the etalon being provided with an optical amplifier. A frequency selection oscillation method comprising: determining a frequency; and selecting one of the standard frequencies by the frequency selection filter. 4. The frequency selective oscillation method according to claim 3, wherein a ring resonator is used as the resonator.