JP2001119086A - Mode hop-free fiber laser device - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To stably oscillate at a single frequency without causing a mode hop and to enable the oscillation wavelength of a fiber laser to be inserted. SOLUTION: By locking the center frequency of a Fabry-Perot filter 8 to the maximum value of the laser output,
By realizing single-frequency oscillation without mode hop and by inserting an element that changes the resonator length of the fiber laser device, the oscillation frequency of the fiber laser device can be accurately extracted without mode hop.

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 single-frequency light from a fiber laser without mode hop.

[0002]

2. Description of the Related Art In recent years, research on fiber lasers that oscillate continuously 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 an excitation light source for exciting the rare earth doped optical fiber, and 3 is an optical coupling for coupling excitation light to the rare earth doped optical fiber. , 4 is an optical splitter that takes out the output, 5 is a polarization-independent optical isolator,
5 'is an optical isolator for suppressing return light to the semiconductor laser, 6 is a polarization controller for compensating the polarization direction during one round, 7 is a Fabry-Perot optical filter having a wide transmission band, 8
Is a Fabry-Perot optical filter having 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 a transmission band composed of a combination of filters 7 and 8,
One of the oscillatable 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 extracted through the optical splitter 4. A Fabry-Perot optical filter is a filter in which two mirrors are arranged to face each other, is a filter that passes only light such that the distance between the mirrors is an integral multiple of the wavelength, and has a plurality of transmission bands.

Here, the oscillation frequency of the laser will be described. The laser cavity length 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
A frequency that is an integral multiple of (nL) is called a longitudinal mode, and is a frequency at which laser oscillation is possible. For example, in FIG. 1, if the resonator length is 10 m and n = 1.5, the longitudinal modes as shown in FIG. 1B exist at intervals of 20 MHz.

[0005] Fabry-Perot filter 7 having a wide transmission band
When a filter having a band of 100 GHz is used, 5000 longitudinal modes can be oscillated in the gain band of the laser. Of these many longitudinal modes, a state in which only one longitudinal mode is oscillating is referred to as “single frequency oscillation”, which uses, for example, a 2 MHz band filter as the Fabry-Perot filter 8 having a narrow transmission band. And can be realized.

FIG. 3 shows this state. 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, only one longitudinal mode cannot be selected because a plurality of transmission bands exist in the gain band of the laser. By using in combination, 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 having a wide transmission band, and a Fabry-Perot filter having a narrow transmission band is used. By selecting the oscillation mode with (mode selection method), oscillation can be performed 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 is shifted to the Fabry-Perot filter. There is a problem that the center of the transmission band of the Perot filter is shifted.

As a result, when the output of the laser fluctuates and the deviation becomes more severe, the oscillation shifts to the adjacent longitudinal mode (mode hop). For this reason, 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 accurately extract the wavelength of the laser, the transmission wavelength of the Fabry-Perot filter 8 does not change, so that only the 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 match. A mode hop occurs.

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

[0012]

According to the present invention, there is provided a ring-type single-frequency oscillation fiber laser device obtained by inserting a Fabry-Perot filter. Provided is a fiber laser device that realizes single-frequency oscillation without mode hop by locking a center frequency to a maximum value of a laser output.

According to the present invention, in order to solve the above-mentioned 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 accurately extracted without mode hop. A fiber laser device according to claim 1, wherein the fiber laser device is configured as described above. In other words, in the locked state, the mode hop does not occur when the fiber laser resonator length is changed, so the mode hop problem that occurs when the fiber length is changed and the oscillation wavelength of the fiber laser is inserted is solved. it can.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments according to the present invention will be described based on embodiments with reference to the drawings.

(Embodiment 1) FIG. 2 is an example of a configuration diagram showing Embodiment 1 of a 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, an excitation light source 2 for exciting the rare-earth-doped optical fiber, an optical coupler 3 for coupling the excitation light to the rare-earth-doped optical fiber, an optical splitter 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 light returning to the pump light source, a polarization controller 6 for adjusting the polarization direction when making a round, and a certain oscillation wavelength. It comprises a Fabry-Perot filter 7 for selecting a mode, and a feedback device 9 for locking a Fabry-Perot filter 8 for selecting a mode and locking the center of its own transmission wavelength in that mode.

When the rare-earth-doped optical fiber 1 is excited by the excitation light source 2 through the optical coupler 3, continuous light oscillates 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 extracted through the optical splitter 4.

Here, for example, the rare-earth-doped optical fiber 1
When an erbium-doped optical fiber is used, the oscillation wavelength of the laser is in the 1.55 mm band. As the excitation light source 2,
1.48 mm semiconductor lasers can be used. The Fabry-Perot filter 7 has a resolution of 1.55 mm
GHz frequency band can be used. As the Fabry-Perot filter 8, a 1.55 mm band having a resolution of 2 MHz can be used. Now, assuming that the resonator length is about 10 m, the oscillatable wavelength (longitudinal mode) determined by the resonator length exists 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 in this, and it is not guaranteed which one oscillates. Therefore, mode hopping frequently occurs, or unstable multi-mode oscillation may occur depending on the band of the Fabry-Perot filter 7.

Therefore, the Fabry-Perot filter 8 uses 25
Select one of the zero vertical modes. However, just by selecting, the selected wavelength is not always at the center of the transmission wavelength of the Fabry-Perot filter 8, and the mode next to the selected one is selected by the Fabry-Perot filter 8. And eventually a mode hop occurs.

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 to the once selected mode, and this mode always oscillates stably.

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

FIG. 3 (a) is observed in the range of about 300 MHz, and the interval shown by the broken line corresponds to 300 MHz. FIG. 3B is an enlarged view of FIG.
Observed in the MHz range. The intervals indicated by broken lines indicate the resolution of the present spectrum analyzer. Fabry
With or without the control of the feedback device 9, the single-frequency oscillation as shown in FIG. 3 is obtained in the short term.

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

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

(Embodiment 2) FIG. 6 is a view for explaining Embodiment 2. In the first embodiment, the wavelength is roughly selected by the Fabry-Perot filter 7, but an optical circulator 10 and a fiber grating 11 may be used as shown in FIG. The optical circulator 10 includes a first circulator as shown in FIG.
The optical element is such that light incident from the port is emitted from the second port and light incident from the second port is emitted from the third port, and an optical element for using a fiber grating, which is a reflective optical element, as a transmission optical element. It is.

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

Third Embodiment FIG. 8 is a diagram showing a third embodiment. In the first embodiment, the wavelength is roughly selected by the Fabry-Perot filter 7, but a dielectric multilayer filter 12 may be used as shown in FIG. Since the dielectric multilayer film type filter can change the wavelength relatively easily, a mode hop-free fiber laser whose wavelength can be easily changed can be realized.

Fourth Embodiment FIG. 9 is a diagram showing a fourth embodiment. In the fourth embodiment, the oscillation frequency of the fiber laser device can be accurately extracted without mode hop by inserting the element A for changing the resonator length in the fiber laser device of FIG. The specific configuration of the element A is a corner cube or a fiber made of PZT.
Wrapped around

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

[0030]

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

When the resonator length of the fiber laser is changed in the locked state, frequency shift without mode hop can be realized. In addition, in this method, although a portion of the output is taken to the feedback device, this requires very little power and does not sacrifice much of the output.

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

[Brief description of the drawings]

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

FIG. 2 is a view illustrating a first embodiment of the present invention, and is a view illustrating 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
(B) is observed in a range of about 30 MHz.

FIG. 4 is a view showing the presence / absence of mode hops, in which the frequency of a mode observed by a Fabry-Perot type spectrum analyzer is observed at regular time intervals.

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

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

FIG. 7 is a diagram illustrating an optical circulator.

FIG. 8 is a view showing a third embodiment of the present invention, and is a view showing a fiber laser using a dielectric multilayer optical filter instead of a Fabry-Perot filter for wavelength selection.

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

[Explanation of symbols]

REFERENCE SIGNS LIST 1 rare earth doped optical fiber 2 excitation light source 3 optical coupler 4 optical splitter 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 Feedback device for realizing precise control 10 Port type optical circulator 11 Narrow band fiber grating 12 Dielectric multilayer film type optical filter A Element for changing resonator length

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hajime Inaba 1-1-4 Umezono, Tsukuba, Ibaraki Pref., National Institute of Metrology (72) Inventor Yoshiaki Akimoto 1-1-4 Umezono, Tsukuba, Ibaraki Pref. 5F072 AB09 AK06 HH02 HH05 JJ05 JJ20 KK07 KK30 LL17 RR01 SS01

Claims (2)

[Claims] In a ring type single frequency oscillation fiber laser device obtained by inserting a Fabry-Perot filter, a mode hop is achieved by locking a center frequency of the Fabry-Perot filter to a maximum value of laser output. A fiber laser device which realizes single-frequency oscillation without noise. 2. The fiber laser device according to claim 1, wherein an oscillation frequency of said fiber laser device is accurately inserted without mode hop by inserting an element for changing a resonator length of said fiber laser device. 2. The fiber laser device according to 1.