JP3371227B2 - Mode hop-free fiber laser device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for generating light of a single frequency from a fiber laser without mode hopping.

[0002]

2. Description of the Related Art In recent years, research on a fiber laser that continuously oscillates at a single frequency has been actively conducted. FIG. 1A shows an example of a conventional single frequency oscillation fiber laser device. In the figure, 1 is an optical fiber doped with a rare earth element (hereinafter referred to as a rare earth doped optical fiber), 2 is a pumping light source for pumping the rare earth doped optical fiber, and 3 is optical coupling for coupling pumping light to the rare earth doped optical fiber. Device, 4 is an optical branching device for extracting the output, 5 is a polarization independent optical isolator,
5'is an optical isolator that suppresses the return light to the semiconductor laser, 6 is a polarization controller that compensates for the polarization direction after one round, 7 is a Fabry-Perot optical filter with a wide transmission band, 8
Is a Fabry-Perot optical filter with a narrow transmission band.

When the rare earth-doped optical fiber 1 is excited by the excitation light source 2 through the optical coupler 3, as shown in FIG. 1 (b), within the transmission band formed by the combination of the filters 7 and 8,
One of the oscillating wavelengths (longitudinal modes) determined by the ring resonator of the fiber laser oscillates in the forward direction of the optical isolator 5. The output light of the laser is taken out through the optical branching device 4. The Fabry-Perot optical filter is a filter in which two mirrors are arranged to face each other, and is a filter that passes only light such that the distance between the mirrors is an integral multiple of the wavelength, and there are multiple transmission bands.

Here, the oscillation frequency of the laser will be described. The cavity length of the laser is L, the refractive index of the optical fiber is n,
If the speed of light is c, the frequency f ₀ = c / determined by the cavity length
The frequency that is an integral multiple of (nL) is called the longitudinal mode, and it is the frequency at which the laser can oscillate. For example, in FIG. 1, assuming that the resonator length is 10 m and n = 1.5, longitudinal modes as shown in FIG. 1B exist at 20 MHz intervals.

Fabry-Perot filter 7 having a wide transmission band
When a filter having a band of 100 GHz is used as 5,000, 5000 longitudinal modes can be oscillated within the gain band of the laser. A state in which only one longitudinal mode oscillates out of these many longitudinal modes is called "single-frequency oscillation". This uses, for example, a 2 MHz band filter as the Fabry-Perot filter 8 having a narrow transmission band. Can be realized.

The state is shown in FIG. In FIG. 3, the horizontal axis represents the frequency (wavelength) of light. When only the Fabry-Perot optical filter 8 having a narrow transmission band is used, since there are a plurality of transmission bands within the gain band of the laser, it is not possible to select only one longitudinal mode, but two Fabry-Perot optical filters are used. Only one longitudinal mode can be selected and the laser oscillates at a single frequency.

That is, in a fiber laser using a rare earth-doped optical fiber, two Fabry-Perot filters having different transmission bands are used, a broad band is determined by a Fabry-Perot filter, and a Fabry-Perot filter having a narrow transmission band is used. By selecting the oscillation mode in (mode selection method), it is possible to oscillate at a single frequency.

[0008]

However, in such a conventional laser oscillator, when the resonator length or the transmission wavelength of the Fabry-Perot filter 8 changes with time due to temperature fluctuations in the laser resonator, the center of the longitudinal mode becomes the Fabry. There is a problem in that the center of the transmission band of the Perot filter deviates.

As a result, the output of the laser fluctuates, and if the deviation becomes more severe, the oscillation shifts to the adjacent longitudinal mode (mode hop). Therefore, the fiber laser periodically repeats mode hopping, and stable single frequency oscillation without mode hopping cannot be obtained.

Further, even if the resonator length of the fiber laser is changed in order to precisely extract the wavelength of the laser, the transmission wavelength of the Fabry-Perot filter 8 does not change, so that only mode hop occurs and the oscillation wavelength does not change. . On the other hand, if the transmission wavelength of the Fabry-Perot filter 8 is changed, the oscillation wavelength changes, but there is no guarantee that the longitudinal mode of the fiber laser resonator and the transmission wavelength of the Fabry-Perot filter 8 will match. Mode hop occurs.

As described above, although it is possible to cause the fiber laser to oscillate at a single frequency in the conventional technique,
Since the resonator length changes with time due to temperature fluctuations and the like, the center wavelengths of the fiber laser resonator and the Fabry-Perot filter 8 do not match, causing mode hopping. That is, it was difficult to stably oscillate at a single frequency without causing mode hop for a long time. Furthermore, it is impossible to subtract the oscillation wavelength of the fiber laser without causing a mode hop.

[0012]

In order to solve the above problems, the present invention provides a ring type single frequency oscillating fiber laser device obtained by inserting a Fabry-Perot type filter, comprising: (EN) Provided is a fiber laser device which realizes single frequency oscillation without mode hop by locking the center frequency to the maximum value of laser output.

According to the present invention, in order to solve the above problems, by inserting an element for changing the resonator length of the fiber laser device, the oscillation frequency of the fiber laser device can be precisely drawn without mode hopping. The fiber laser device according to claim 1 is provided. In other words, since the mode hop does not occur when the fiber laser cavity length is changed in the locked state, the problem of the mode hop that occurs when the oscillation wavelength of the fiber laser is changed by changing the cavity length is solved. it can.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described based on examples with reference to the drawings.

(Embodiment 1) FIG. 2 is an example of a configuration diagram showing Embodiment 1 of the mode-hop free fiber laser device of the present invention. In FIG. 2, the fiber laser device includes a rare earth-doped optical fiber 1, a pumping light source 2 for pumping the rare earth-doped optical fiber, an optical coupler 3 for coupling pumping light to the rare earth-doped optical fiber, an optical branching device 4 for extracting an output, A polarization-independent optical isolator 5 that suppresses light returning in the opposite direction, an optical isolator 5 ′ that prevents return light to the pumping light source, a polarization controller 6 for adjusting the polarization direction after one round, and an oscillation wavelength to some extent. It is composed of a Fabry-Perot filter 7 to be selected and a feedback device 9 for locking a Fabry-Perot filter 8 which selects a mode and locks the center of its transmission wavelength in the mode.

When the rare earth-doped optical fiber 1 is excited by the excitation light source 2 through the optical coupler 3, continuous light oscillation occurs in the forward direction of the optical isolator 5 within the transmission band of the Fabry-Perot filter 7. The output light of the laser is taken out through the optical branching device 4.

Here, for example, the rare earth-doped optical fiber 1
When an erbium-doped optical fiber is used as the laser, the oscillation wavelength of the laser is 1.55 mm band. As the excitation light source 2,
A 1.48 mm semiconductor laser can be used. The Fabry-Perot filter 7 has a resolution of 1.55 mm band of 5
GHz ones can be used. The Fabry-Perot filter 8 having a resolution of 2 MHz in the 1.55 mm band can be used. Now, assuming that the resonator length is about 10 m, oscillating wavelengths (longitudinal modes) determined by the resonator length exist at intervals of about 20 MHz.

The transmission band of the Fabry-Perot filter 7 is 5
If it is GHz, there are 250 longitudinal modes of the fiber laser, and it is not guaranteed which one oscillates. For this reason, mode hop frequently occurs, and unstable multimode oscillation may occur depending on the band of the Fabry-Perot filter 7.

Therefore, the Fabry-Perot filter 8 has 25
Choose one of the 0 vertical modes. However, the selected wavelength does not always mean that the selected wavelength is always at the center of the transmission wavelength of the Fabry-Perot filter 8, and the adjacent mode that is intended to be selected is selected by the Fabry-Perot filter 8. There is a mode hop after all.

Therefore, the output from the fiber laser is constantly monitored by the feedback device 9, and the Fabry-Perot filter 8 is controlled so as to maximize the output. Then, the center of the transmission wavelength of the Fabry-Perot filter 8 is always locked once in the selected mode, and this mode always continues stable oscillation.

Next, the operation of the present invention will be described. FIG. 3 shows a state of single frequency oscillation of the fiber laser shown in FIG. 2 observed with a Fabry-Perot type spectrum analyzer. The vertical axis represents intensity and the horizontal axis represents oscillation frequency. This Fabry-Perot type spectrum analyzer is 1.3 μm
It shows a superposition of a large number of spectra in which the wavelength region of ~ 1.7 μm is divided every 300 MHz.
300 M if the fiber laser oscillates at a single frequency
There is only one mode in Hz.

FIG. 3A was observed in the range of about 300 MHz, and the interval indicated by the broken line corresponds to 300 MHz. FIG. 3B is an enlarged view of FIG.
Observed in the MHz range. The interval indicated by the broken line shows the resolution of this spectrum analyzer. Fabry
Whether the Perot filter 8 is controlled by the feedback device 9 or not, a single frequency oscillation as shown in FIG. 3 can be obtained in the short term.

In FIG. 4, the oscillation wavelength observed by the Fabry-Perot type spectrum analyzer as shown in FIG.
It was observed at a certain time interval. FIG. 4A shows a case where the feedback device 9 is not operated.
(B) is the one when operated. In FIG. 4A, it can be seen that the mode hops to the adjacent mode in the band of the Fabry-Perot filter.

Also, when observed with a Fabry-Perot type spectrum analyzer, although basically a single frequency oscillation as shown in FIG. 3 is performed, mode hops occur irregularly, and at that moment, FIG. It is observed that it is causing a burst like. On the other hand, no mode hop is observed at all in FIG. 4B, and when observed by the Fabry-Perot spectrum analyzer, the state is always as shown in FIG. That is, in this example, it can be seen that the fiber laser stably oscillates at a single frequency.

(Second Embodiment) FIG. 6 is a diagram for explaining the second embodiment. Although the wavelength is roughly selected by the Fabry-Perot filter 7 in the first embodiment, an optical circulator 10 and a fiber grating 11 may be used as shown in FIG. As shown in FIG. 7, the optical circulator 10 has a first
The optical element is such that the light incident from the port is emitted from the second port and the light incident from the second port is emitted from the third port, and an optical element for using the fiber grating which is a reflective optical element as a transmissive optical element. Is.

Unlike the Fabry-Perot filter, the fiber grating has no repetition of the transmission band and has a function of cutting incoherent light, so that a fiber laser having further excellent stability can be realized.

(Third Embodiment) FIG. 8 is a diagram showing a third embodiment. Although the wavelength is roughly selected by the Fabry-Perot filter 7 in the first embodiment, the dielectric multilayer filter 12 may be used as shown in FIG. Since it is relatively easy to change the wavelength of the dielectric multilayer filter, it is possible to realize a mode-hop-free fiber laser that can easily change the wavelength.

(Fourth Embodiment) FIG. 9 is a diagram showing a fourth embodiment. In the fourth embodiment, the element A for changing the cavity length is inserted in the fiber laser device shown in FIG. 2 so that the oscillation frequency of the fiber laser device can be precisely drawn without mode hopping. The specific configuration of the element A is a corner cube or a PZT fiber.
It is made by winding it around.

Although the embodiments of the present invention have been described with reference to the first to fourth embodiments, the present invention is not limited to such embodiments, and various modifications can be made within the scope of the technical matters described in the claims. It goes without saying that there are various embodiments.

[0030]

As described above, according to the present invention,
In the fiber laser, the central wavelength of the narrower Fabry-Perot filter of the two filters is locked to one of the longitudinal modes of the fiber laser by feedback control, so that a high-power single polarization single mode without mode hop is generated. A single-frequency oscillation fiber laser can be realized.

Further, if the resonator length of the fiber laser is changed in the locked state, frequency insertion without mode hop can be realized. In addition, with this method, a part of the output is taken by the feedback device, but this requires very little power, so that the output is hardly sacrificed.

Further, according to the present invention, mode hop does not occur even if the cavity length is changed, so that the present invention can be applied to a laser capable of precisely inserting and extracting a wavelength.

[Brief description of drawings]

FIG. 1 is a configuration example of a conventional single-frequency oscillation fiber laser using two etalons, and (b) illustrates the principle of the optical wavelength (frequency) on the horizontal axis.

FIG. 2 is a diagram showing Example 1 of the present invention, and is a diagram showing a single-polarization single-frequency oscillation fiber laser using a polarizer.

FIG. 3 shows a state of single frequency oscillation observed by a Fabry-Perot type spectrum analyzer. (A) is about 3
Observed in the range of 00 MHz, (b) is observed in the range of about 30 MHz.

FIG. 4 is a graph in which the presence or absence of a mode hop is observed, and the frequency of the mode observed by a Fabry-Perot spectrum analyzer is observed at regular time intervals.

FIG. 5 shows the presence or absence of mode hop, which is observed with a Fabry-Perot spectrum analyzer.

FIG. 6 is a diagram showing a second embodiment of the present invention and is a diagram showing a fiber laser using an optical circulator and a fiber grating instead of the Fabry-Perot filter for wavelength selection.

FIG. 7 is a diagram illustrating an optical circulator.

FIG. 8 is a diagram showing a third embodiment of the present invention, and is a diagram showing a fiber laser using a dielectric multilayer film type optical filter in place of the Fabry-Perot filter for wavelength selection.

FIG. 9 is a diagram showing a fourth embodiment of the present invention and is a diagram showing a configuration in which an element for changing the cavity length is inserted in the fiber laser device.

[Explanation of symbols]

1 Rare-earth-doped optical fiber 2 Pumping light source 3 Optical coupler 4 Optical brancher 5 for extracting output 5 Polarization-independent optical isolator 5'Optical isolator 6 Polarization controller 7 Fabry-Perot optical filter 8 Fabry-Perot optical filter 9 Mode hop-free For realizing various controls 10 Port type optical circulator 11 Narrow band fiber grating 12 Dielectric multilayer optical filter A Element for changing resonator length

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hajime Inaba 1-4 Umezono, Tsukuba-shi, Ibaraki Industrial Technology Institute (72) Inventor Yoshiaki Akimoto 1-4 1-4 Umezono, Tsukuba-shi, Ibaraki Industrial Technology Institute In the Institute of Metrology (56) Reference JP-A-5-175577 (JP, A) JP-A-5-299758 (JP, A) JP-A-9-139536 (JP, A) JP-A-10-125983 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01S 3/00-3/30

Claims (2)

(57) [Claims] 1. A ring type single frequency oscillation fiber laser device obtained by inserting a Fabry-Perot type filter, wherein the center frequency of the Fabry-Perot type filter is
Control so that the output from the balaser is always the maximum value
As a result, a single-frequency oscillation without mode hop is realized, which is a fiber laser device. 2. An oscillation frequency of the fiber laser device can be precisely inserted without mode hopping by inserting an element for changing the resonator length of the fiber laser device. 1. The fiber laser device according to 1.