JP2949977B2 - Tunable optical fiber laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength tunable optical fiber laser using a rare earth doped optical fiber.

[0002]

2. Description of the Related Art Some rare earth-doped optical fibers are excited by light of a specific wavelength and have gain. It is well known that oscillation occurs when feedback is applied to a gain medium, and an optical fiber laser can also oscillate based on this principle. Such a technology is described in, for example, 1991 IEICE Spring National Convention C-31.
6 and JP-A-2-43782.

Next, the configuration of a wavelength tunable optical fiber laser according to the prior art will be described with reference to FIG. 2, 21 is a light source module for excitation, 22 is a wavelength division multiplexing optical fiber coupler,
23 is an output plug, 24 is a rare earth-doped optical fiber, 25
Is an optical isolator and 26 is a wavelength variable bandpass filter.

The wavelength λ emitted from the light source module 21
The light a enters the rare-earth-doped optical fiber 24 after passing through the wavelength-division-multiplexed optical fiber coupler 22, and excites the rare-earth-doped optical fiber 24. Excited rare earth doped optical fiber 2
4 emits spontaneous emission light in the gain wavelength band Δλ. The emitted light in the wavelength band Δλ goes around the closed circuit of the optical isolator 25 for limiting the traveling direction of the light → the wavelength tunable bandpass filter 26 → the wavelength multiplexing optical fiber coupler 22 → the rare earth-doped optical fiber 24, and the rare earth as the gain medium Optical feedback is applied to the added optical fiber 24 to cause laser oscillation.

The wavelength tunable bandpass filter 26 has a wavelength band Δλ.
Can be changed within the range. By changing the wavelength of the light fed back to the rare earth-doped optical fiber 24, the oscillation wavelength of the optical fiber laser can be changed. That is, if the passing wavelength of the wavelength variable bandpass filter 26 is set to λb, the oscillation wavelength of the optical fiber laser becomes λb. Part of the laser-oscillated light is extracted from the output plug 23 through the wavelength multiplexing optical fiber coupler 22.

[0006]

FIG. 2 shows a problem that a part of the pumping light of wavelength .lambda.a emitted from the pumping light source module 21 leaks from the wavelength multiplexing optical fiber coupler 22 to the output plug 23 side and becomes a noise component. There is. This is due to crosstalk of the wavelength division multiplexing optical fiber coupler 22. About 5% of the light having the wavelength λa incident from the light source module 21 leaks to the output plug 23 side. When taking out the laser-oscillated light of wavelength λb to the output plug 23 side,
The crosstalk of the wavelength division multiplexing optical fiber coupler 22 is used.

As described above, in addition to the light of the wavelength λb that has been laser-oscillated toward the output plug 23, the excitation light of the wavelength λa leaks and becomes a noise component. For example, if the output of the excitation light of wavelength λa emitted from the excitation light source module 21 is 50 mW,
About 2.5 mW is emitted to the output plug 23 side.
If the output level of laser-oscillated light of wavelength λb is 5 mW, the output level difference between these two wavelengths is 3 dB, which is a very large noise component. SUMMARY OF THE INVENTION It is an object of the present invention to provide a low-noise tunable optical fiber laser in which pump light does not leak to an output terminal.

[0008]

According to the present invention, there is provided an excitation light source for emitting light having a wavelength of λa.
A reflection type optical filter 3a that transmits light from the excitation light source 1 and reflects light in the wavelength band Δλ; a rare earth-doped optical fiber 4 having a wavelength band Δλ as a gain wavelength band;
a optical filter 3b that transmits the light of the wavelength band a and reflects the light of the wavelength band Δλ; an optical isolator 5 that receives the light of the wavelength band Δλ reflected by the reflective optical filter 3a;
a wavelength-selective optical element 6 having wavelength selectivity in the range of λ,
Output mirror 7 to which the output of wavelength selective optical element 6 is input
And forming an optical loop that goes around the reflection type optical filter 3a, the optical isolator 5, the wavelength selective optical element 6, the output mirror 7, the reflection type optical filter 3b, and the rare earth doped optical fiber 4 in this order. Outgoing light of wavelength λa;
The light in the wavelength band Δλ that travels around the optical loop is made to travel oppositely inside the rare-earth-doped optical fiber 4, and the light in the wavelength band Δλ is extracted.

[0009]

Next, the configuration of a wavelength tunable optical fiber laser according to the present invention will be described with reference to FIG. 1 is an excitation light source,
2a, 2b and 2c are lenses, 3a. 3b is a reflection type optical filter, 6 is a wavelength selective optical element, 7 is an output mirror, 1
Reference numerals 1, 12, and 13 denote parallel light, 14 denotes output light, and the other components are the same as those in FIG. 2.

In FIG. 1, light emitted from an excitation light source 1 having a wavelength λa is converted into parallel light 11 by a lens 2a, and is incident on a reflection type optical filter 3a. The reflection type optical filter 3a has a wavelength λa.
, And reflects light in the wavelength band Δλ. The parallel light 11 that has passed through the reflection type optical filter 3a is condensed by the lens 2b, is incident on the rare earth-doped optical fiber 4, and excites the rare earth-doped optical fiber 4. Excitation light of wavelength λa that has passed without being absorbed by the rare-earth-doped optical fiber 4 is converted into parallel light 12 by the lens 2c and transmitted through the reflective optical filter 3b. The reflection type optical filter 3b transmits light of wavelength λa and reflects light of wavelength band Δλ. The rare-earth-doped optical fiber 4 excited by the light having the wavelength λa emits spontaneous emission light in the gain wavelength band Δλ. The emitted light in the wavelength band Δλ is
b, the light is converted into parallel light 13 and the reflection type optical filter 3a →
The optical isolator 5 for limiting the traveling direction of the light → the wavelength selective optical element 6 → the output mirror 7 → the light is condensed by the lens 2c via the reflection type optical filter 3b, and reenters the rare earth-doped optical fiber 4. Then, optical feedback is applied to the rare-earth-doped optical fiber 4 to cause laser oscillation.

The wavelength-selective optical element 6 can change the wavelength within the range of the wavelength band Δλ. By changing the wavelength of the light fed back to the rare-earth-doped optical fiber 4, the oscillation wavelength of the optical fiber laser can be changed. That is, if the wavelength of the wavelength selective optical element 6 is set to λb,
The oscillation wavelength of the optical fiber laser is λb. Part of the laser-oscillated light is extracted from the output mirror 7 and becomes the output light 14.

[0012]

Next, an embodiment of FIG. 1 will be described. Excitation light source 1
Has an oscillation wavelength of 1.48 μm and an optical output of 10 mW to 10 mW.
Using a 0 mW semiconductor laser, the reflection type optical filter 3
a.3b transmits 1.48 μm band light and 1.55 μm.
An interference filter that reflects light in the m band is used. An erbium-doped optical fiber is used for the rare-earth-doped optical fiber 4, and a wavelength-selectable optical element 6 is a wavelength-variable band-pass filter capable of changing a passing wavelength in a 1.55 μm band and having a half-value width of about 3 nm; Alternatively, a diffraction grating having a high diffraction efficiency in the 1.55 μm band is used. The output mirror 7 has:
A laser mirror having a reflectance of about 90% and a transmittance of about 10% in a 55 μm band is used.

The light having a wavelength of 1.48 μm from the excitation light source 1 is converted into parallel light 11 by the lens 2a,
After passing through a, these are arranged so as to be coupled to the erbium-doped optical fiber by the lens 2b. Then, a wavelength of 1.48 μm that has passed without being absorbed by the erbium-doped optical fiber.
The light of m is converted into parallel light 12 by the lens 2c, and the reflection type optical filter 3b is disposed in front of the light.

In order to return the light in the 1.55 μm band emitted from the excited erbium-doped optical fiber to the erbium-doped optical fiber again and to apply optical feedback, the reflection type optical filter 3a, the optical isolator 5 and the wavelength selectivity are required. It is necessary to adjust the optical axis by the optical element 6, the output mirror 7, and the reflection type optical filter 3b. Thus, a wavelength tunable optical fiber laser oscillating at a single wavelength in the 1.55 μm band can be manufactured. When changing the oscillation wavelength, the setting of the wavelength-selective optical element 6 is changed.

In the configuration shown in FIG. 1, erbium-doped light is used to prevent oscillation due to reflection at both end faces of the erbium-doped optical fiber and to realize a low-noise tunable optical fiber laser in which pump light does not leak to the output end. 1.55 μm band and 1.48 μm after obliquely polishing both ends of the fiber 4 degrees
An antireflection coat having a reflectance of 0.5% or less is applied to both of the bands.

[0016]

According to the present invention, it is possible to provide a low-noise tunable optical fiber laser in which pump light does not leak to the output terminal. Further, the difference between the output level at two wavelengths, that is, the output level at which the excitation light of the wavelength λa emitted from the excitation light source is emitted to the output side and the output level of the laser light of the wavelength λb can be 40 dB or more. Thus, it is possible to prevent the excitation light from being superimposed on the single-wavelength output light 14 and causing noise.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a wavelength tunable optical fiber laser according to the present invention.

FIG. 2 is a configuration diagram of a wavelength tunable optical fiber laser according to the related art.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 excitation light source 2a lens 2b lens 2c lens 3a reflection optical filter 3b reflection optical filter 4 rare earth doped optical fiber 5 optical isolator 6 wavelength selective optical element 7 output mirror 11 parallel light 12 parallel light 13 parallel light 14 output light

Claims (1)

(57) [Claims] An excitation light source for emitting light having a wavelength of λa.
A reflection type optical filter (3a) that transmits light from the excitation light source (1) and reflects light in the wavelength band Δλ; a rare earth doped optical fiber (4) having the wavelength band Δλ as a gain wavelength band; A reflective optical filter (3b) that transmits light of the wavelength band Δλ and reflects light of the wavelength band Δλ reflected by the reflective optical filter (3a); A wavelength-selective optical element (6) having wavelength selectivity in the range of band Δλ, and an output mirror (7) having an output of the wavelength-selective optical element (6) as an input, and a reflective optical filter (3a) , Optical isolator (5),
An optical loop is formed around the wavelength-selective optical element (6), the output mirror (7), the reflection type optical filter (3b), and the rare-earth-doped optical fiber (4) in this order, and the output of the wavelength λa from the excitation light source (1). A wavelength tunable optical fiber laser characterized in that the emitted light and the light in the wavelength band [Delta] [lambda] traveling around the optical loop are made to travel oppositely inside the rare-earth-doped optical fiber (4) to extract the light in the wavelength band [Delta] [lambda].