JPH0736463B2 - Hikari Baraman Laser - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical fiber Raman laser, and more particularly to a compact and highly efficient wavelength tunable laser.

[Prior art]

FIG. 1 is a conceptual diagram of a conventional device of this type,
a is a YAG laser with Q switch and mode lock device,
1-b is a drive device of 1-a, 2 is a lens for condensing a laser beam, 3 is a single mode optical fiber, 4 is a spectroscopic device, and 5 is an object to be measured.

In order to operate this, operate the drive device 1-b,
For example, Q switch frequency 1KHz, mode lock frequency 100M
An optical pulse having a large peak output of Hz is extracted from the YGA laser 1-a, and is incident on the single mode optical fiber 3 via the lens 2. Since the optical output in the fiber at this time is 300 W or more at the peak output, the stimulated Raman phenomenon occurs in the core portion of the fiber 3, and the optical output is produced in the wavelength region of, for example, 1.06 to 1.5 μm. This light is dispersed by the spectroscopic device 4 and is incident on the required object 5 to be measured through the remeter lens 2. Further, when it is desired to select the wavelength of the light source, the optical pulse output can be obtained in the above wavelength range by driving the spectroscopic device 4.

[Problems to be Solved by the Present Invention]

Such a conventional device has a high output as shown in FIG.
A YAG laser 1-a is used to obtain a wavelength tunable laser output, which has a wide application range for various measurements including optical fiber optics.

However, the YAG laser used in this type of device is generally a water-cooled system and has a resonance length of 2 m or more, and it was necessary to operate it on a large surface plate for stable use. For this reason, it has been difficult to incorporate this type of laser into the device or make it portable.

[Object of the Invention]

An object of the present invention is to generate an extremely high power density inside the oscillation medium of an optical fiber Raman laser using a single mode optical fiber doped with a rare earth as an oscillation medium, and to generate an extremely high power density inside the optical fiber by the oscillation line peculiar to the laser beam. Stimulated Raman scattering is generated in the core part of the above, and Stokes lines due to stimulated Raman scattering are extracted from the fiber laser in addition to the laser oscillation line, and a compact and highly efficient portable wavelength tunable laser is provided.

[Means for solving problems]

In order to achieve the above object, the present invention provides a resonator system including a laser oscillation medium formed of a single mode optical fiber doped with a rare earth element and a resonance mirror that causes laser resonance, and a resonator system that emits light to the resonator system. An optical fiber Raman laser for incidence, comprising a light source for exciting a rare earth element, wherein the maximum optical power Pm in the laser oscillation medium is at least [Where Pc is the threshold of stimulated Raman scattering, A is the cross-sectional area (cm ² ) of the optical fiber core, and G is the Raman gain (cm /
W), α are optical attenuation coefficients of optical fiber cm ⁻¹ ), and 1 is fiber length (cm). ] It is characterized by having an excitation light source output as described above.

In particular, while using a resonant mirror formed directly on the end face of a single-mode optical fiber having a minute core, the efficiency of the laser is increased and the function of the resonator system as a single-mode optical waveguide is maintained. In order to provide a modulator for mode lock and Q switch, and to improve the function of modulation, the optical fiber itself, which is the laser oscillation medium, has 10
^-It is characterized in that the stimulated Raman scattering effect can be generated in the resonator system of the optical fiber Raman laser by maintaining the birefringence of ^-4 or more.

In the present invention, since the Raman effect is not directly generated by the laser output light, an optical nonlinear effect (stimulated Raman scattering) can be generated in the resonator system in which optical power of several tens of times the laser output exists. The light source for exciting the laser can be downsized. As a result, for example, a semiconductor laser can be used for excitation, and a compact, lightweight, highly efficient wavelength tunable laser can be provided.

[Embodiment 1] FIG. 2 is a conceptual diagram of an embodiment of the present invention, in which 6 is an output.
GaAs laser (light source) with 100 mW and oscillation wavelength of 0.83 μm, 7 is a rod lens for condensing, 8 is a resonant mirror made of a dielectric material deposited on the end face of the optical fiber, and 9 is Nd about 80 in the core.
ppm-doped panda-type single-mode optical fiber with birefringence, 10 is LiNbO ₃ waveguide phase modulator, 11 is
LiNbO ₃ directional-coupled optical modulator, 12 is a dielectric resonant mirror with vapor-deposited 11 on one side, 13 is laser output light, 14 is Ge
A photodetector, 15 is a driving oscillator for 10, 10 is a frequency divider for dividing the frequency of the output signal of 15 at a constant rate (for example, 100 or 1/1000), and 17 is a frequency divider 16 Oscillator for oscillating at output frequency and driving 11, 18 is a sampling oscilloscope for observing laser oscillation light pulse, 19
Is a trigger signal for waveform observation.

The reflectance of the resonance mirror 12 is a laser oscillation wavelength of 1.09 μm.
To about 90%, and about 5% to 0.83 μm of the excitation light source. The transmittance at a wavelength of 1.09 to 1.3 μm was 10 to 20%.

On the other hand, the reflectance at the incident end 8 is about 99.5% for a laser oscillation wavelength of 1.09 μm and about 1% for a pumping light source wavelength of 0.83 μm.

To operate this, a stable drive current of 12 is required for the GaAs laser 6.
0 mA is made to flow, and the obtained laser output is made incident on the single mode optical fiber 9 via the rod lens 7. Ga at this time
The output of the As laser 6 is 110 mW and the coupling efficiency of the fiber 9 is 5
It was 5%. In this state, Nd was excited by the incident light to oscillate the laser light between the reflecting mirrors 8 and 12, and 15 mW was obtained as the continuous oscillation output.

Next, the driving oscillator 15, the frequency divider 16, and the directional coupler (optical modulator) 11 driving oscillator 17 of the phase modulator 10 are driven to perform the Q switch and the mode lock operation. At this time, the oscillator 15 causes the mode lock operation,
Drive frequency fm Given in. Where l ₁ is the length of the single mode fiber
n ₁ is the refractive index of the core of a single mode fiber, l ₂ is LiNbO ₃ variator two general, n ₂ is the refractive index of LiNbO _3, C is the speed of light. In this example, l ₁ = 103 m, n ₁ = 1.456, l ₂ = 5 cm, n ₂
= 2.23, the phase modulator 10 is driven at fm = 1.00 MHz. On the other hand, the oscillator 17 performs the Q switch operation, and its driving frequency is 1/1000 of fm, that is, fQ =
1KHz was selected. With respect to changes in fm and fQ due to changes in test conditions, the optimum frequency is adjusted by fine adjustment of the reference signal generator incorporated in the driving oscillator 15.

3 (a) and 3 (b) are detailed diagrams of the optical modulators of the phase modulator 10 and the directional coupler 11, respectively, and 20 is LiNb.
A substrate of O ₃ (thickness 1.2 mm, length and width 25 mm × 20 mm), 21 is an optical waveguide portion (6 μm width) in which Ti is diffused, and 22 is an electrode for applying a driving voltage.

In this directional-coupling type optical modulator 11, the coupling between the two optical waveguide portions 21 changes corresponding to the driving frequency fQ applied to the electrode 22, which changes the Q of the resonator system. .

In the connection between the single-mode optical fiber and each optical modulator, fine adjustment was performed so that the principal axes of the polarization planes of the optical fiber and the optical modulator coincided with each other, and they were adhesively fixed with an epoxy resin. The optical insertion loss of the optical modulators 10 and 11 is 1.09 μm.
At 1.8 and 1.6 dB, respectively.

By the above operation, the optical modulators 10 and 11 were operated, and the mode-locked and Q-switch optical pulses were obtained as the output of the laser output light 13. The obtained light pulse having a wavelength of 1.09 μm was 640 sec in the case of only the Q switch operation, and 3.5 nsec in the case of simultaneously performing the Q switch and the mode lock.

FIG. 4 shows output characteristics obtained by dispersing the output light 13 by the diffraction grating type spectroscope. The wavelength resolution of the spectrometer is 50Å.

As shown in Fig. 4, wavelengths outside the fundamental oscillation line of 1.09 μm
Broad optical output characteristics were obtained at 1.15 μm and 1.21 μm. This is because the maximum optical power Pm in the laser oscillator is the threshold value Pc of stimulated Raman scattering due to the pumping light output from the pumping light source (GaAs laser 6) [Reference: proceedings of IEEE,
Vol.68, NO.10, PP.1232-1236 (1980)], that is, Since stimulated Raman, stimulated Raman scattering occurs in the optical fiber 1
1.15 μm of secondary Stokes light and 1. of secondary Stokes light
21 μm of light was generated. However, in the equation (2), A is the cross-sectional area [cm ² ] of the optical fiber core, G is the Raman gain [cm / W], and α is the optical attenuation coefficient [cm ^-1 ] of the optical fiber.
l is an optical fiber length [cm]. In this embodiment, since the core diameter is 5 μm, A = 1.96 × 10 ⁻⁷ cm ² and G is an approximate value in the case of silica glass. (Λ is the wavelength of light [μm]), G = 9.17 × 10 ⁻¹² [cm / W], and l is the length of the single-mode optical fiber, 1.03 × 10 ⁶ [c
m], and the fiber loss value is 1.2 dB / km at a wavelength of 1.09 μm, so α = 2.76 × 10 ⁻⁶ [cm ⁻¹ ].

Therefore, the calculated value of Pc in equation (2) is 34 [W].
To get

In the case of the operation of only the Q switch, the optical output in this example was 1500 mW as the peak optical output. Therefore, the virtual peak light output Po when the Q switch and the mode lock are simultaneously operated is estimated to be about 15W. By the way, this optical output of 15 W is the value when the reflectance of the resonant mirror is 90%, so the maximum optical power in the laser oscillation medium at this time, that is, in the single-mode optical fiber, is
Pm reaches about 150W. This is the most important point of the present invention. This in-fiber output of 150 W greatly exceeds the threshold value 34 [W] of stimulated Raman scattering,
It is explained that the Stokes light of FIG. 4 is observed.

Summarizing the above, the conditions that cause stimulated Raman scattering in a fiber Raman laser are: Becomes

Here, Po is the peak output of the laser oscillation medium when the Q switch and the mode lock operation are performed, and R is the reflectance of the emitting side resonance mirror. However, in this Po, when stimulated Raman scattering occurs in the optical fiber, the laser oscillation output decreases due to the generation of Stokes light,
Virtual light output (the fiber laser output value obtained assuming that stimulated Raman scattering does not occur),
It should be noted that the output is not actually directly observable. That is, in the case of this example, the actually observed oscillation output at a wavelength of 1.09 μm is 7.2 W, which is below the virtual value of 15 W. This difference corresponds to the conversion of light energy into the generation of Stokes light, and
The observation results in the figure support this.

The birefringence of the single mode optical fiber used in this example was B = 5 × 10 ⁻⁴ . This birefringence is 1 ×
At 10 ^-4 , the Q-switch and the optical modulator used for the mode-lock operation do not operate efficiently, which causes a problem that the modulation degree cannot be deep. Therefore, the condition of B> 1 × 10 ⁻⁴ (4) is required.

In this embodiment, the core is made of Nd-doped fiber, but rare earth elements such as Er, Ho, and Yb capable of lasing can be used as the doping material.

[Embodiment 2] FIG. 5 is a schematic diagram of an optical modulator of LiNbO ₃ used in the second embodiment, in which 23 is an electrode for a high frequency signal (several MHzHz to several KHz) and 24 is a low frequency. Electrodes for signals (several hundred MHz to several KHz), 25
Is a dielectric vapor deposition mirror (resonant mirror) for forming a laser resonator system, and has the same structure as that of the first embodiment except for the above. In this embodiment, the modulator 10 in the first embodiment is used.
The parts corresponding to and 11 are integrated in one element shown in FIG. That is, a high frequency signal for mode lock is applied from the electrode 23, and a low frequency signal for Q switch is applied from the electrode 24. The modulators used here are all directionally coupled LiNbO ₃ modulators. The insertion loss of this modulator was 1.9 dB at a wavelength of 1.09 μm. As described above, the reason why the insertion loss is reduced in the present embodiment is that the number of connecting portions is reduced in two places as compared with the first embodiment.

The Raman output similar to that shown in FIG. 4 was obtained in this example as well, but the loss of the resonance system is small, so that the intensity of the first Stokes line (λ = 1.15 μm) is 30% as compared with the first example. It was confirmed to increase.

[Third Embodiment] FIG. 6 is a block diagram of another optical modulator for performing a Q-switch operation, instead of the directional coupling type optical modulator 11 shown in the third embodiment. Is a Nd-doped single mode optical fiber for constructing an optical fiber laser resonator system, 26 is a laminated piezoelectric oscillator, 27 is an AC voltage generator for driving 26, and 28 is a fiber support.

To operate this modulator, the Nd-doped single mode optical fiber that constitutes the resonator system is cut to form an end face perpendicular to the axial direction. As the forming method, the cleaved surface of the fiber is used or the direction of smooth polishing is used. In addition, anti-reflection coating is applied on both sides.
As shown in FIG. 6, the two end faces of this optical fiber are fixed to the optical fiber support base 28 with an adhesive or the like, and the other end is fixed to the laminated piezoelectric vibrator. In this state, the air gap d in FIG. 6 is set to 5 μm or less, preferably 1 to 2 μm, and mutual positional fine adjustment is performed so that the optical coupling loss between both end surfaces is 0.2 dB or less. In this state, when an AC voltage having a peak voltage of about 105 V is applied from the AC voltage generator 27, the optical fiber 9 vibrates in the direction of the arrow in FIG. The vibration width at this time was ± 5 μm. It was confirmed that the optical coupling efficiency between both end faces of the optical fiber was changed between 0% and 96% by this vibration.

FIG. 7 shows an embodiment in which the above modulator is incorporated in an optical fiber laser resonator system, and the structure is almost the same as that of the first embodiment except this modulator. However, modulator 10
A resonance mirror 12 is vapor-deposited on one surface of the. When the optical fiber Raman laser shown in FIG. 7 was operated in the same procedure as in the first embodiment, an optical spectrum similar to that in FIG. 4 was obtained. In the present embodiment, the insertion loss of the modulator shown in FIG. 6 is 0.2 dB and the insertion loss of the modulator 10 is 1.2 dB, which is a total.
It was 1.4 dB. As a result, the second Stokes light (λ = 1.1
It was confirmed that the light intensity of 5 μm) increased to 2.3 times that of the first embodiment.

[Embodiment 4] In the embodiment shown in FIG. 7, the GaAs laser light source 6 was replaced by a dye laser having a wavelength of 0.69 μm and an output of 3 W, and the same experiment as in the above embodiment was conducted. However, here the resonance mirror 8
The reflectance at wavelengths of 0.69 μm and 1.09 μm is about 1% and 99.5%, respectively.
And the reflectance at 1.09 μm is about 15% and
90%. The transmittance of the mirror 12 at a wavelength of 1.09 to 1.6 μm was 5% or more.

In this example, since the excitation input is large, the stimulated Raman light should be observed up to the third order (1.28 μm), the fourth order (1.36 μm), the fifth order (1.44 μm), and the sixth order (1.55 μm). I was able to.

The single mode fiber used in the present invention uses a panda type birefringent fiber, but is not necessarily limited to the panda type, and may be an ellipse, a jacket type, a bow tie type, a side tunnel. Any type of birefringent fiber such as shape can be applied.

〔The invention's effect〕

As described above, according to the present invention, it is possible to configure a highly efficient fiber Raman laser, and by using strong oscillation light in the laser resonator system, a configuration that causes stimulated Raman scattering. Therefore, it is possible to excite the laser with a semiconductor laser or a small portable laser. Therefore, since it is possible to provide a portable wavelength tunable laser with extremely small size and high efficiency, not only it can be used as a light source for characteristic evaluation of an optical fiber, but also it has an excellent advantage that it can be applied to various optical measurements.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a conventional device of this type, FIG. 2 is a configuration diagram of a first embodiment of the present invention, and FIG. 3 is a detailed diagram of an optical modulator used in the first embodiment. FIG. 4 is an output optical spectrum of the first embodiment, FIG. 5 is an optical modulator for mode lock and Q switch used in the second embodiment, and FIG. 6 is Q used in the third embodiment. FIG. 7 is a block diagram of a switch optical modulator, and FIG. 7 is a block diagram of a third embodiment. 1-a ... YAG laser with mode lock and Q switch,
1-b ... Driving device, 2 ... Convex lens, 3 ... Single-mode fiber, 4 ... Spectroscopic device, 5 ... Object to be measured, 6 ...
GaAs laser, 7 ... Rod lens, 8 ... Resonant mirror,
9 …… Nd-doped panda type single mode optical fiber, 10 …… Li
NbO ₃ phase modulator (optical modulator), 11 …… LiNbO ₃ directional coupler (optical modulator), 12 …… Resonant mirror, 13 …… Laser output light, 14 …… Ge photodetector, 15 …… 10 driving oscillators, 16…
… Divider, 17 …… 11 drive oscillator, 18 …… Sampling oscilloscope, 19 …… Trigger signal, 20 …… LiNbO ₃
Substrate, 21 ... Ti-doped optical waveguide, 22,23,24 ... Electrode, 25 ... Resonant mirror, 26 ... Stacked piezo oscillator, 27
...... AC voltage generator, 28 …… Fiber support.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroyuki Suda Inventor Hiroyuki Suda 162 Shirahone, Shikatakata, Tokai-mura, Naka-gun, Ibaraki Nippon Telegraph and Telephone Corporation, Ibaraki Electro-Communications Research Laboratory (56) References Proceedings of the IEEE, vol ． 68, No. 10 (1980-10) (US) P. 1232-1236

Claims (5)

[Claims] 1. A resonator system comprising a laser oscillation medium composed of a single-mode optical fiber doped with a rare earth element and a resonance mirror for causing laser resonance, and a rare earth element for injecting light into the resonator system. An optical fiber Raman laser composed of a light source for exciting an element, wherein the maximum optical power Pm in the laser oscillation medium is at least [Where Pc is the threshold of stimulated Raman scattering, A is the cross-sectional area (cm ² ) of the optical fiber core, and G is the Raman gain (cm /
W), α are optical attenuation coefficients of optical fiber cm ⁻¹ ), and 1 is fiber length (cm). ] An optical fiber Raman laser having the pumping light source output as described above. 2. The laser oscillation medium of the resonator system is 1 × 10 ⁻⁴
A single mode fiber having the above birefringence,
And the resonator system is of one type or two that causes one or both of the Q-switch and mode-lock operations.
The optical fiber Raman laser according to claim 1, characterized in that the optical fiber Raman laser comprises an optical modulator of a kind. 3. The resonator thin film mirror of claim 1, wherein the resonator mirror is a dielectric thin film mirror formed directly at an input end and an output end of the resonator system.
An optical fiber Raman laser according to the item. 4. In the optical modulator, a single mode optical fiber constituting a resonator system is smoothly divided into two in a direction perpendicular to a central axis of the optical fiber, and two end faces of the optical fiber are mutually separated. Has a parallel gap within 5 μm and is arranged so that the central axes of the optical fibers are aligned with each other, and an element for vibrating the optical fibers in a radial direction at a constant frequency is provided on one or both of the optical fibers. The optical fiber Raman laser according to claim 2, wherein the optical fiber Raman laser is provided. 5. The Q-switch and the mode-locking optical modulator are waveguide type LiNbO ₃ optical modulators in which two kinds of electrodes for low frequency and high frequency are mounted on a single substrate. The optical fiber Raman laser according to claim 2, characterized in that