JPH05299758A - Frequency-stabilized ring laser - Google Patents

Abstract

PURPOSE:To prevent the oscillation frequency of an optical ring laser from being fluctuated by an external disturbance and to stably oscillate the optical ring laser when the optical ring laser is oscillated in a single longitudinal mode at a narrow spectral-line width. CONSTITUTION:One part of the laser oscillation output of a ring laser is injected into a ring in a direction opposite to that of its laser oscillation; a laser oscillator is utilized as a resonator in a light path in the opposite direction. A circulator 4 is installed in the ring; a difference is formed in a light-path length in a direction opposite to the positive direction; an-attenuator 5 is installed in the light path in the opposite direction; a slight difference is formed in an optical loss. A beam of light in the opposite direction is detected; an error signal is obtained by means of a comparator 14; a laser-resonator length is fixed by using a phase modulator 15 by means of a feedback by the error signal. By its result, a laser-oscillation wavelength is stabilized without a need for the absolute standard of a wavelength. Consequently, an oscillation frequency is stabilized with reference to an external disturbance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention provides an optical ring laser with a narrow spectral line width in order to meet the need for a light source having a stable frequency and a narrow spectral line width with the recent development of communication. The present invention relates to a frequency-stabilized ring laser in which the oscillation frequency is prevented from fluctuating due to disturbance when oscillating in a single longitudinal mode at all times, and stable laser oscillation is possible.

[0002]

2. Description of the Related Art A ring laser oscillates with a narrow spectral line width, and since it is a traveling wave type, it has the advantage that single mode oscillation is easy. There have been some reports on laser oscillation by doving rare earths such as erbium ions into an optical fiber or a quartz waveguide.

[0003]

However, in the laser oscillation by the above-mentioned conventional ring laser, the laser resonator length slightly fluctuates due to vibration, acoustic noise, temperature fluctuation, etc., and as a result, the oscillation wavelength is continuous or discontinuous. It was unavoidable that it changed. With respect to the ring laser, there is no attempt to stabilize the oscillation wavelength so far because the development history is short. On the other hand, there are attempts to stabilize the oscillation wavelength of semiconductor lasers, but in order to suppress changes in the oscillation wavelength, another resonator made of a material with a small temperature coefficient is used so that the resonance wavelength becomes extremely stable. It has been necessary to have a complicated structure in which it is used as a reference or an absolute frequency reference based on the absorption line of an atom is used, and the laser cavity length is controlled by feedback. Moreover, there are few reports of such frequency stable lasers.

Further, a discontinuous change in oscillation wavelength has been a problem. In particular, since the optical fiber ring laser has a long optical resonator length, the free spectral range of resonance is short, and therefore, there are many optical resonance modes within the gain band in which the optical resonator can oscillate. When some disturbance is applied to such a laser resonator, the oscillation mode jumps randomly to another mode. That is, the oscillation wavelength changes discontinuously. In fact, even if vibrations and acoustic noises were removed and temperature fluctuations were sufficiently prevented, it was not possible to avoid a mode jump about once every few seconds. The most reliable method for preventing such mode jump is to use a bandpass optical filter having a narrow pass band within the longitudinal mode interval of the resonator in the laser resonator. However, there has been no bandpass optical filter having a performance suitable for such a purpose, so that the mode jump of the fiber ring laser cannot be avoided.

The present invention has been made in order to solve the above-mentioned problems, and an object of the present invention is to provide an optical ring laser with a narrow spectral line width in a single longitudinal mode in which the oscillation frequency is disturbed. Another object of the present invention is to provide a frequency-stabilized ring laser that prevents fluctuations due to fluctuations and enables stable oscillation.

[0006]

In order to achieve the above object, in the frequency stabilized ring laser of the present invention, a ring laser for oscillating a laser is provided by providing an optical amplifying means in an optical ring resonator. A portion of the light that lases the ring in one direction at any point in the ring of the ring resonator is extracted and the other light in the ring is tuned to orbit the ring in the opposite direction. The optical circulator has a path for injecting from one point, and has an optical circulator in the ring, and the optical circulator traces when light incident on the optical circulator from the one direction is emitted.
At least partly differs from the second optical path traced when the light incident on the optical circulator from the opposite direction is emitted, and the optical path length between the first optical path and the second optical path is different. There is a difference and a slight difference in optical loss, and further, a part of the light that circulates in the opposite direction of the ring is extracted from the ring, and a change in intensity thereof is used as an error signal to form a common optical path portion of the ring. It is characterized by having a feedback circuit that expands and contracts the optical path length.

[0007]

In the frequency-stabilized ring laser of the present invention, a part of the laser oscillation output of the ring laser is injected into the ring in the direction opposite to that of the laser oscillation, and the laser oscillator is also used as a resonator in the optical path in the opposite direction. To do. An optical isolator is provided in this ring to provide a difference in the optical path length between the forward and reverse directions and a slight difference in the optical loss in the reverse optical path. The light in the reverse direction is detected, an error signal is obtained from the difference between the mode behaviors of the forward direction and the reverse direction, and the laser resonator length is controlled and fixed by the feedback circuit based on the error signal. The result is that the laser oscillation wavelength is stabilized without the need for an absolute wavelength reference, thus stabilizing the oscillation frequency against disturbances.

[0008]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a basic configuration diagram showing a first embodiment of the present invention. In the figure, 1 is an antireflection end, 2 and 3 are couplers, 4 is a circulator, 5 is an attenuator, 6 and 7 are mirrors, 8 is a pump light source, 9 is a dichroic mirror, 10 is an active fiber, 11 is a filter, 12 Is an isolator, 13 is a light receiver, 14 is a comparator, 15 is a phase modulator,
Reference numeral 16 is a DC value, 17 is a coupler for extracting a laser output, and 18 is a laser output.

In the configuration of this embodiment, the dichroic mirror 9 and the active fiber 1 are provided in the fiber ring.
0, the filter 11, the coupler 2, the coupler 3, the phase modulator 15, and the circulator 4 are sequentially arranged counterclockwise to form a resonator loop, and the excitation light source 8 is coupled to the dichroic mirror 9. At the clockwise exit end of the coupler 2,
Join the antireflection edges. A coupler 17 for extracting a laser output 18 and an isolator 12 are sequentially coupled to a counterclockwise emission end of the coupler 3, and an output of the isolator 12 is coupled to a clockwise incidence end of the coupler 2. on the other hand,
A light receiver 13 is coupled to the clockwise output end of the coupler 3. The output of the light receiver 13 is connected to one input of the comparator 14, the DC value 16 is connected to the other input of the comparator 14, and the output of the comparator 14 controls the phase modulator 15. The circulator 4 serves to send different paths in clockwise and counterclockwise directions. In the clockwise direction, the path reflected by the mirror 6 via the attenuator 5 is sent, and in the counterclockwise direction, the mirror is sent. The path reflected at 7 is assumed to be a sensation.

The spontaneous emission light excited by the excitation light source 8 and emitted from the active fiber 10 follows one path of the filter 11, the couplers 2 and 3, the phase modulator 15 and the circulator 4, passes through the dichroic mirror 9, and is amplified again. Repeatedly, laser oscillation is generated counterclockwise and output 1
It is taken out from the coupler 3 as 8. Tuning of the oscillation wavelength can be performed by setting the pass band of the filter 11.

Here, a part of the laser light is the isolator 1.
It returns to the fiber coupler 2 via 2 and then propagates clockwise in the resonator loop. This light enters the circulator 4 again, but the path that the light follows inside the circulator 4 at this time is different from the path in the clockwise loop.
Since the circulator 4 is designed so that the optical path length to follow depends on the path, the resonator length also differs between clockwise and counterclockwise. Therefore, the free spectrum range (hereinafter, referred to as FSR) of both is also different depending on the difference in the cavity length.
Moreover, for example, the attenuator 5 is used only in one of the paths that the light follows inside the circulator 4 so that the light loss in the circulation is slightly different in both directions. In this way, when a part of the laser light is taken out and again made incident on the same resonator in the reverse direction (clockwise in FIG. 1) again, if the optical loss in the reverse direction is slightly large, the resonator is moved in this direction. Operates as a bandpass filter for incident laser light without oscillating laser.

To operate the resonator as a bandpass filter having high finesse without oscillating the resonator, the attenuator 5 may be set to a value just before the resonator oscillates. Regarding this technology, Japanese Patent Application No. 3-102 has already been applied.
No. 924.

Here, in the optical path of the resonator, the portions in which the light in both forward and reverse directions travels in different optical paths are stable against disturbance, and in this portion, the optical path length between the forward and reverse directions of the resonator is long. The difference shall not change. On the other hand, the phase modulator 15 provided in the portions where the bidirectional lights travel in the same optical path in the opposite directions respectively expands or contracts the optical length of the fiber or the waveguide in this portion by an electric signal.

The operation of the embodiment having such a configuration will be described below.

In the present embodiment, when the longitudinal mode of laser oscillation and the pass mode of the resonator (which operates as a bandpass filter in the opposite direction) in the opposite direction match, the resonator (bandpass filter) The laser light is detected by the output of). If the frequencies of the two modes do not match, no laser light is detected at the output of the resonator (bandpass filter). Therefore, when the frequencies of the two modes are completely the same, the resonator (bandpass filter) output shows a maximum value, and the output decreases with the frequency difference between the two modes. This output has a concave curve with respect to the frequency difference.
Therefore, a value lower than this maximum value is set as the target value, the voltage value corresponding to this target value is given by the signal generator, and the difference from the photoreceiver output is used as the error signal, and the optical path length of the common optical path part of the fiber ring is set. It is possible to fix the optical output to the target value by constructing a feedback circuit that feeds back the expansion and contraction of. Fixing the laser output to a certain value in such a control system is nothing but fixing the frequency of laser oscillation. However, it is difficult to fix the system at the peak point of the concave curve because the sign of the error signal is not determined. One embodiment for realizing such control is shown in FIG. Here, the laser oscillation power detected by the light receiver 13 is compared with the DC value 16, and the error signal is compared with the phase modulator 1
It has a circuit configuration that feeds back to 5.

Next, it will be explained below that the frequency of laser oscillation can be fixed by fixing the laser output to a certain value in the control system having the above feedback circuit.

Here, as a result of the disturbance being applied to the resonator, only the optical path portion shared by the resonators expands and contracts according to the disturbance among the optical paths forming the resonators in both the forward and reverse directions. To do. On the other hand, it is assumed that an optical path portion that is not shared by the resonators, in other words, a portion that each resonator independently has as a separate optical path is sufficiently protected from disturbance and does not expand or contract. In this case, the change in the optical path length of each resonator is the same. As the resonator length expands and contracts, the passing wavelength of each resonator changes accordingly. The variation width Δλi of the passing wavelength (i = 1, 2, 3, ... Corresponds to the numbers of the respective resonators) is defined by the free spectral range F of each resonator.
SRi (i = 1, 2, 3, ...), If the expansion / contraction length common to each resonator is δ and the wavelength is λo,

[0019]

## EQU1 ## Δλi = FSRi × δ / λo. Here, even if δ is constant, Δλi depends on FSRi (depending on each resonator length). This shows the following for the embodiment of the present invention (FIG. 1). That is, in the embodiment of the present invention, when the laser wavelength oscillating in one direction matches one of the passing wavelengths of the resonator (bandpass filter) in the opposite direction, if this resonator length changes by δ, The laser wavelength and the pass wavelength condition of the bandpass filter no longer match. Therefore, the output light power from the bandpass filter changes according to the difference between the laser wavelength and the passing wavelength of the bandpass filter. Therefore, the control for fixing the output light power from the bandpass filter to a constant value corresponds to the control for fixing the resonator length (control for setting δ = 0).

FIG. 2 illustrates this. (1)
Indicates that the laser wavelength oscillating in one direction matches one of the passing wavelengths of the resonator (bandpass filter) in the opposite direction, and a large reception output power is obtained.
(2) is the case where the resonator length changes by δ = λ / 2 (just half the wavelength) at the common portion, and the laser wavelength and the bandpass filter are both shifted by a frequency corresponding to 1/2 of the FSR. To do. As a result, the center frequencies of the two are displaced, and the reception output power is lower than that in (1). This change in the reception output power gives a control error signal.

This control accuracy is improved as the spectral line width of laser oscillation is narrower and more stable, and as the finesse of the ring resonator in the reverse direction is higher. At this time, if there is a slight difference in the optical path lengths around both sides of the resonator, a slight expansion and contraction of the common optical path portion can significantly change the light receiving level of the light receiver 13, and a large error signal for control can be obtained. However, it should be noted that if the laser oscillation power itself changes due to a change in pumping light power, a change in optical loss of the resonator, or the like, the error signal also changes.

FIG. 3 shows a concrete structure of the circulator 4 in the embodiment of the present invention, which is 4-1 and 4
-2 is an input / output fiber, 4-3 is an upright birefringent crystal, 4-
4 is a 45 degree Faraday rotator, 4-5 is a 45 degree tilt birefringent crystal, 4-6 is an optical path difference providing part, 4-7 is an attenuator (see FIG. 1).
Of the attenuator 5) and 4-8 are mirrors (mirrors 6 and 7 of FIG. 1).
Equivalent to). Details of this operation are already in Japanese Patent Application No. 3-2.
Since it is disclosed in No. 20505, detailed description thereof will be omitted here. In short, the inside of the circulator 4 is shown in FIG.
[The optical path part that each resonator independently has as a separate optical path] described above, but by constructing this in a bulk type, it is possible to avoid the influence of temperature changes and other disturbances. This is possible and suitable for achieving the object of the present invention.

FIG. 4 is a block diagram showing a second embodiment of the present invention. This embodiment has exactly the same configuration as that of the first embodiment except that the laser output is taken out differently, and the same components are denoted by the same reference numerals as those in FIG. In this embodiment, the laser output 18 'is taken out from the clockwise emission end of the coupler 2.

This embodiment uses a clockwise filter output indicated by 18 'as a laser output, and one coupler (17 in FIG. 1) can be omitted as compared with the configuration in FIG. 1, which is efficient. is there.

Although FIG. 1, FIG. 3 and FIG. 4 show the embodiments based on the optical fiber resonator, basically the same is true even if an optical resonator using an optical waveguide is used. Configurable. Regarding laser resonator technology using optical waveguides,
Japanese Patent Application No. 63-173938, Japanese Patent Application No. 1-71828
And Japanese Patent Application No. 3-281579. When a laser resonator using an optical waveguide is provided with the feedback mechanism of the present invention, as the phase modulator 15, a thermal strain type phase modulator that thermally expands and contracts the waveguide can be used. As described above, the present invention is variously applied and can take various embodiments in accordance with its purpose.

[0026]

As is apparent from the above description, according to the frequency-stabilized ring laser of the present invention, when the optical ring laser always oscillates in a single longitudinal mode with a narrow spectral line width, its oscillation frequency causes disturbance. Fluctuation can be prevented and stable oscillation becomes possible.

In the past, in order to stabilize the frequency of laser oscillation, complicated control that required another frequency reference was required. However, in the present invention, by using one's own laser resonator as the frequency reference, There is no need for a separate frequency reference external to the laser cavity. In the present invention, the oscillation wavelength can be tuned, and the wavelength once determined can be fixed with a simple configuration. Such lasers are extremely effective in communication, measurement, and other fields.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a process of generating an error signal for control according to the present invention.

FIG. 3 is a diagram showing a specific configuration example of a circulator in the first embodiment.

FIG. 4 is a configuration diagram showing a second embodiment of the present invention.

[Explanation of symbols]

 1 ... Antireflection end 2, 3 ... Coupler 4 ... Circulator 5 ... Attenuator 6, 7 ... Mira 8 ... Excitation light source 9 ... Dichroic mirror 10 ... Active fiber 11 ... Filter 12 ... Isolator 13 ... Photodetector 14 ... Comparator 15 ... Phase modulator 16 ... DC value 17 ... Coupler 18 ... Laser output

Claims (1)

[Claims] 1. A ring laser for performing laser oscillation by providing optical amplification means in an optical ring resonator, wherein one point of light lasing the ring in one direction at any one point in the ring of the optical ring resonator. And a path for injecting this light from another point in the ring so as to circulate the ring in a direction opposite to the one direction, and an optical circulator in the ring, The circulator has a first optical path traced when the light incident on the optical circulator from the one direction is emitted, and a second optical path traced when the light incident on the optical circulator from the opposite direction is emitted. At least a part of the optical path is different from the optical path, and there is an optical path length difference between the first optical path and the second optical path and a slight optical loss difference. Further, the ring is circulated in the opposite direction. Light A frequency-stabilized ring laser having a feedback circuit for extracting a part of the optical path length from the ring and expanding / contracting the optical path length of the common optical path part of the ring by using the variation of the intensity as an error signal.