JP2659013B2 - Stimulated emission device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an active device based on the principle of optical amplification by a fiber optic laser amplifier (so-called stimulated emission or stimulated emission). A device that operates both as a laser oscillator and as an amplifier, and is referred to in the present invention as a photostimulated emission device). This improvement or related improvements are those in which the effective gain is provided by stimulated emission of radiation. Type of active device. These devices include a length of fiber in which the distribution of active dopant ions is deployed longitudinally;
An optical pumping source coupled to the fiber.

BACKGROUND OF THE INVENTION Hybrid semiconductor-diode and optical fiber technologies are both well advanced. However, full integration of these technology elements should still be achieved. There is a need for active devices that can be easily incorporated into single-mode fiber systems and bridge these technologies.

Details of lasers in the form of multimode fibers have been published early in the last decade (1973). As background, important reading books, j.Stone and CABur
The following editorial is by rus: That is, "Neodymium-
dopedsilica lasers in end-pumped fiber geometr
y ", Appl Phys Lett 23pp388-389 (Oct1973); and" N
eodymium−Doped Fiber Lasers: Room Temperature cw O
peration with an Injection Laser Pump ", Appl Optic
s13pp1256-1258 (June 1974). The laser
It is stated to consist of a stub of predetermined length fiber (about 1 cm long) with a core ranging from 800 to 15 microns in diameter. Continuous wave (cw) laser operation has demonstrated the use of gallium arsenide (GaAs) injection lasers with coupled ends as optical pumping sources.

[Problems to be Solved by the Invention] Despite considerable progress in single-mode fiber technology, more than 10 years have passed since then, and fiber lasers that are fully compatible with single-mode fiber systems up to now. Has not been developed.

The present invention has been made in order to solve the above-mentioned problem, and an object of the present invention is to provide a photostimulated emission device used as a fiber laser amplifier compatible with an end-pumping communication optical fiber system in single mode operation. I do.

[Means for Solving the Problems] That is, the present invention is an optical fiber optical stimulated emission device which is a type of active device in which gain is provided by radiation and excitation light emission, comprising a core and a clad, Consists of a silica glass optical fiber (1) containing active dopant ions and an optical pump source (11) connected to the optical fiber (1), and transmits light in a single mode along the fiber (1). The optical pump source (11) that propagates and combines to excite the light emitted therefrom is by a so-called side pump (9).

That is, a feature of the present invention is an active device including a core containing an active dopant of a rare-earth transition element, a clad, an end-pumped glass optical waveguide, and obtained by stimulated emission of light. The optical waveguide is a length of silica glass fiber for communication, the fiber having a geometry that propagates a single mode of light, the active dopant is an element selected from the group of erbium, terbium, And a light-stimulated emission that is contained and dispersed at a low concentration along the length of the fiber to provide stimulated emission by a three-level system, wherein the core has low propagation loss communication fiber characteristics. In the device.

The fiber (1) has a length of at least 5 cm, is of single mode geometry and allows to maintain single transverse mode propagation at the emission wavelength, the active dopant ions being rare earth transition elements, 90
The fiber (1) at a low level of uniform concentration of less than 0 ppm
Wherein the fiber (1) has an extremely low propagation loss.

[Operation] In the stimulated emission optical fiber device of the present invention, it is convenient to use the single-mode fiber, and the fiber has a step index profile having a step in the bending ratio. However, other forms of single mode fiber are not excluded from the scope of the invention and may be in the form of polarization preserving fiber, polarizing fiber, dispersion shifted fiber and helical fiber and helical core fiber. Daughter pants can be included in the fido core, fiber cladding, or both. One or more dopant types can be included to increase device diversity.

Further, many passive fiber devices can be used with the active device to provide spectrum control or temporary switching.

(1) Diffraction grating. Removing the fiber cladding by chemical or mechanical means exposes the field and deposits a grating on the fiber. Thereby, a narrow spectral line of the laser can be obtained.

(2) Acousto-optic modulation of either vertical or horizontal fiber lasers. Use piezoelectric materials or sound waves. This allows switching and modulation to be achieved, including cavity Q-switching and mode locking.

(3) What about fiber polarizers? It can be used to control the output polarization state either with an exposed field device or with the use of a single polarization fiber.

New manufacturing process (SBPool et al “Fabrication of
low-loss optical containing rare-earth ions ", Ele
By using cctron Lett 21pp737-738 (1985)), it is now possible to construct single mode fibers with uniform low dopant concentrations up to 900 ppm, and modern telecommunications fibers (ie, less than 40 dB / km, typically Typical value is ~ 1dB / km), which maintains the low loss characteristic. These fibers include fused couplers, polarizers, filters and phase modulators (eg, single polarization operation,
Fiber devices (to achieve wavelength selection, modelocking and Q-switching) and are therefore fully compatible with the existing all-fiber laser /
It is possible to envision amplifier technology.

As can be considered, a light-stimulated emission device represented by a single mode fiber laser amplifier has a number of advantages over bulk counterparts. Due to its small core (typically less than 8 μm in diameter), very low threshold values (100100 μW) and high gain can be obtained. Also, the overall diameter of the typical fiber is about 100μ.
m and the thermal effect has been found to be minimal.

As a result of these attributes, effective laser action or amplifier gain can be generated for uncommon rare earth transition metal dopants and uncommon optical transitions (optical transitions). Optical transitions) are inherently weak. It has been found that continuous laser operation at room temperature is possible with a three-level laser system that previously operated only with a pulse motor.

Since the dopant concentration is low, the production can be economical. A typical device is 0.1 of doped oxide.
It will be used as small as μg.

A laser / amplifier or photostimulated emission device with the lowest dopant concentration, as considered herein, is relatively long, e.g.,
It is said to combine fibers that are 5cm and at least larger than 300m (where the length of the fiber acts as a filter for the cladding working mode and the gain is dispersed), making a compact device be able to. A wound 1m long fiber laser
Can be incorporated within a volume of 1 cm ³ .

Choosing silica as the fiber host medium has good power handling characteristics. In addition, a glass host with a silica content will substantially widen the ion optical transition of the rare earth or transition metal dopant. This will enable both a tunable laser and a broadband amplifier to be implemented, as will be described later.

Most rare earth and some transition metal pants have been experimentally investigated. There is a window for all of them, where the losses are low despite being very close to the high loss absorption band. This allows for very long amplifier and laser structures. Lasers 300m in length are being tested.

In summary, the active devices provide an improved source / amplifier for telecommunications applications because they can handle high power without damage, and they are small / light / inexpensive general It can be used with other fiber devices (e.g., diffraction gratings) that provide active devices for various purposes and provide new and powerful signal processing functions. Also, non-linear effects can be easily obtained at the optical power levels obtained in fiber lasers, and many laser and non-linear effects can be exploited.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

Here, referring to the drawing, Nd ³⁺ is doped with 300 ppm,
A single mode geometry (single mode geometry) step index profile fiber of germania / silica core with 1 μm cut-off wavelength and 1% index difference is disclosed in U.K.
It was constructed using the method described in US Pat. No. 520,300 and is described in the aforementioned article "Fabrication of low-loss optical
Fibers containing rare-earth ions "is summarized in detail. In summary, neodymium chloride, a high-purity hydrate, is used as a dopant source during a modified vapor deposition (NCVD) process. The trichloride is first dehydrated in the presence of a chlorine staining gas and melts on the walls of the carrier chamber.

In silica communication fibers, it is customary to add germanium to increase the refractive index between the sheath and the core by a desired amount. However, unlike in this case, germanium atoms are replaced by silicon atoms in the structure of glass, so that the presence of germanium does not cause a significant change in light transmission characteristics. The results of this embodiment are suitable for telecommunications applications, and doping with a rare earth material to make a laser amplifier (photo-pumped emission device) is the same as pure silica fiber.

Glass material for cladding, such as SiO ₂ -B ₂ O _3, is deposited on the inner wall of a silica tube heated by a conventional method.
Is done. Thereafter, the carrier chamber is preheated to about 1000 ° C. to obtain the desired pressure. During core deposition, the reactants (typically GeCl ₄ and SiCl ₄ ) are mixed with oxygen so that a controlled amount of the rare earth halide is suspended in the gas stream and heated. Pass through the chamber. The temperature of the MCVD hod zone is chosen to be insufficient to melt the deposited core glass layer. This essential feature allows the material to be further dried before melting. This latter drying step is performed by prolonged heating at 900 ° C., or, for example, in the presence of a stream of chlorine gas. The unsintered layers are those that are melted to form a glassy layer, the preform collapses, and the fibers are drawn into the prescribed shape.

Fiber 1 is 2m long, pump wavelength 820nm, 5dB / m
Was selected to have an absorption effect of The loss at the laser working wavelength (1.088 μm) of this fiber was 4 dB / km. FIG. 1 shows the configuration of the experiment. Fiber end
3,5 are cleaved, high reflectivity at laser wavelength (~ 99.5%)
And directly attached to the insulating mirrors 7, 9 which have a high transmission (~ 80%) to the pump. In order to increase the accuracy of the cavity, it is essential to minimize the angle of the fiber end, thereby ensuring close contact with the mirror. A cleavage tool from York Technology with tool type number 007 was used, and the fiber ends 3,5 were examined before index matching with the mirrors 7,9. Alternatively, a reflective coating, for example a multilayer dielectric coating, may be deposited directly on the fiber end face.

The pumping from the end (end pumping) is a single mode GaAlAs laser 11 which is an optical pump source.
(Hitachi HLP 1400), which is a lens
Focused by 13,15 and delivered into fiber with 16% efficiency. The laser action threshold was observed for a total semiconductor laser power of 600 μW. This corresponds to an estimated absorption pump power of only 100 μW in the 2 m long fiber 1 and represents a very low intracavity loss.

The output power as a function of the pump power for the fiber laser is shown in FIG. The saturation of the output was observed up to the maximum effective pump power (20 mW). The operation of the laser at reduced duty cycle did not decrease at the laser action threshold, indicating that the effects of heat would be ignored. The fiber laser, unlike the prior art neodymium-doped glass laser, can easily operate CW without auxiliary cooling. From 300 cavity treatments, the modulation of the pump caused relaxation oscillations, from which the cavity accuracy 300 was calculated.

The wavelength of operation of the fiber laser was measured to be 1.088 μm, ie shifted by a wavelength of about 30 μm, which is longer than expected for a conventional neodymium glass laser.

It is noted that a fiber grating can be used at the end mirror 9 and can be spliced or formed in the doped fiber (1).

The laser cavity described above can be modified to provide a Q-switching function. A typical device is shown in FIG.
Is shown in Here, the microscope objective lens 17,
An acousto-optic deflector (acoustic optic deflector) 19 and an output mirror 21 are used at the position of the contact mirror 9 in FIG. In the configuration tested, the used fiber 1
Had the following properties: core diameter 3.5 μm, 0.21
The total absorption at NA, 3.2 m in length and at the pump wavelength (contents correspond to 300 ppm Nd ³⁺ ) was 98%. The loss at the laser working wavelength (1.088m) can be ignored (10dB / km)
Was something. The fiber ends 3 and 5 were cleaved, and one end 3 was in contact with an input insulating mirror 7. This mirror 7
While having high transmission at the pump wavelength (T = 85%) and high reflectivity at the laser operating wavelength (R = 99.8%), low reflectivity mirrors can also be used to advantage. The pump source 11 is used as described above, and light is pumped into the fiber with approximately 25% efficiency. CW threshold is 3.7m
W absorption. The acousto-optic modulator 19 was used in transmission mode, and the high Q state was obtained by electronically removing the RF supplied with a 2 μs duration pulse. The output mirror 21 used in this configuration had a transmission at the laser working wavelength of 12%. The pulse repetition rate was variable between a single shot and 4 kHz where the peak output power or pulse duration peak did not change.

A mechanical chopper with a mark space ratio of 1: 300,
As an alternative to Q-switching, the cavity was replaced. By using an output mirror 21 with a transmission of 65% at the laser working wavelength, an output pulse with a peak power of 300 mW or more and a repetition rate of 400 Hz of 500
NS FWHM was obtained at that time. Saturable absorbers can also be used for Q-switching and mode-locking. This could be incorporated into the fiber as an additional dopant.

As shown in FIG. 4, a beam splitter 23 and a reflection diffraction grating 25 can be added to the Q-switching device of FIG.

In a test experimental setup, 15 dB / unsaturated absorption at 514 nm
A 5 m long, Nd ³⁺ doped fiber 1 having a length of m was used as the gain medium. An argon ion laser was used as a pump source. The optical (optical) feedback is a flat input mirror (R>99%＠1.09μm;
T = 80% ＠ 514 nm) and a diffraction grating 25 (600 rows / nm, braced at 1 μm). An in-cavity thin film (pellicle) was used as the beam splitter / output coupler 23. The laser working wavelength can be selected by changing the angle of the diffraction grating 25. The laser was widely tunable and tunable over a range of 80 nm from 1065 nm to 1145 nm. The threshold occurred at an input of 25 mW corresponding to an absorption of only 10 mW in the fiber. Pulsed and cw operation was shown.

Other rare earth or transition metal dopants can be incorporated into the fiber using the techniques described above with the use of certain halide dopant precursors. They also show a high absorption band at the practical pump wavelength and a low loss window at the effective emission wavelength. This is shown in FIG. 5 for rare earth erbium (Er ³⁺ ) and terbium (Tb ³⁺ ).

Similarly, the device of FIG. 4 was tested on erbium-doped fiber. Fiber 1 is 51
It had an unsaturated absorption of 10 dB / m at 4 nm and a length of 90 cm.
This is because the input dielectric mirror 7 (R=82%＠1.54 μm; T = 7
7% (＠ 514 nm). Further, a diffraction grating of 600 rows / mm was braced at 1.6 μm. 1.528 to 1.542 and 1.5
The entire tuning range of 25 nm from 44 to 1.555 μm is the threshold (30 m
W) was obtained with three times the pump power (see FIG. 6). This is comparable to the wavelength range relevant for long section (long hole) fiber optic communications. Praseodymium (also called praseodymium: Pr) doped fibers can also be tuned. Using a CW Rh6G dye laser at 590 nm and a 1 μm diffraction grating, an adjustment range of 1048 nm-1109 nm 61 nm
was gotten. The threshold occurred at an absorption power of 10 mW.

The length of the fiber 1 can be coiled to provide a compact package. It is possible to construct a ring cavity laser structure,
An example is shown in FIG. Here, the fiber ring 27
(70 cm in diameter) was made by splicing two ports of a fused tapered coupler 29 made from Nd ³⁺ doped fiber. The coupler 29 has 595n
More than 80% of the power of the dye laser pump 31 at m was designed to couple to the ring 27, and less than 10% per round trip was extracted at the laser working wavelength. The loss of the coupler was measured to be 3 dB at 633 nW (fiber multi-mode), and 1 μm
Was 1dB. Although the fiber is the one used in the previous embodiment (FIG. 1),
The absorption of the 5 nm dye laser pump wavelength is quite high (30 dB /
m). In this ring laser structure, the pump radiation must be largely absorbed by the ring 27, not the fiber 1. Thus, it is advantageous to construct the coupler 29 from one undoped fiber and one doped fiber, this lead does not absorb at the pump wavelength. The lasing threshold is observed at 80 mW dye laser pump power, which corresponds to a few milliwatts absorbed in the ring due to coupler loss and lead absorption. At a maximum dye laser power of 280 mW (assuming a ring absorption of 20 mW), the ring laser output was 2 mW. When considered for a bidirectional ring laser output, the tilt efficiency is approximately 20%.

The laser working wavelength has a maximum full width at half maximum (FWHM) spectrum of 4 nm and is centered at 1.078 μm (see FIG. 9). The 10 nm shift from the linear configuration is due to a slight mismatch between the coupler wavelength response and the laser gain curve. It therefore makes it possible to adjust the laser wavelength over the entire gain curve (90 nm width) by changing the coupler characteristics.

The amplifier is shown in FIG. 10, which is constructed using an open-ended four-port coupler 29 consisting of doped fiber 1. This is inserted into a conventional transmission fiber 35 as shown in FIG. At the emission wavelength, a signal is transmitted along the main fiber 1, which acts as a gain medium. The pump radiation from the source 31 is connected to the coupling fiber 33 and from there to the main fiber 1.

Increased efficiency can be obtained using a coupler designed to couple the pump radiation into the fiber 1 but not at the emission wavelength. In addition, the leads 33 are used to ensure that the majority of the pump power is absorbed in the selection of the amplification fiber 1.
It would be beneficial to have an undoped fiber.

25dB single pass gain, 3m long using similar equipment
Erbium-doped fiber (300ppm Er ²⁺ )
Was measured for To prevent the onset of laser action, the optical feedback resulting from the fluorescent reflection is reduced by flexibly matching one end of the fiber. In practice, splicing the fiber in a fiber system is sufficient to largely eliminate the etalon effect, since low reflectivity connections can easily be achieved.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide a fiber laser and an amplifier thereof that are compatible with a single mode operation and a single mode fiber system.

Erbium and terbium (and praseodymium)
Is a convenient dopant for silica glass. The reason is that these elements exhibit relatively large light absorption at practical pump wavelengths and have low loss windows at useful light emission wavelengths. Erbium is particularly advantageous because it is a three-level system, unlike the neodymium four-level system, and although it cannot be easily pumped by a flashlamp, its 1.55 μm laser wavelength is more than that of silica glass communication fibers. It is in perfect alignment with the window with the lowest loss. Furthermore, it can be easily pumped with low power semiconductor diodes, and its gain characteristics provide various advantages.

The silica glass used in this specification includes not only pure water silica but also characteristics of silica glass suitable for communication,
That is, it should be understood that it includes silica which is not so large as to significantly degrade the properties such as low loss and splicing properties, but is co-doped with a small amount of a material having an effect of modifying the refractive index. Such materials include:
In addition to germania mentioned in the examples, there are phosphorus pentoxide, boron trioxide, alumina and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic plan view showing the configuration of a diode-pumped fiber Fabry-Perot laser, FIG. 2 is a graph showing the output power as a function of the measured pump power of the laser of FIG. 1, and FIG. FIG. 4 is a schematic plan view showing the structure of a Q-switch cavity laser which is different from the laser of FIG.
5 is a schematic plan view showing the configuration of a tunable cavity laser different from that of FIG. 5, FIG. 5 is a graph showing the absorption of the spectrum of a silica fiber doped with neodymium, erbium, and terbium, and FIGS. 6 and 7 are erbium. FIG. 8 shows the emission spectra of praseodymium-doped fibers and the tuning response of lasers containing these fibers; FIG. 8 is a schematic plan view showing the configuration of a fiber ring cavity laser pumped with a dye laser; FIG.
Is a graph showing the output spectrum of the ring cavity laser shown in FIG. 8, and FIG. 10 is a schematic plan view showing a doped fiber amplifier.

1 ... fiber, 3, 5 ... fiber end, 7, 9 ... insulating mirror, 11 ... optical pump source (single mode GaAlAs laser), 13, 15 ... lens, 17 ... microscope objective lens, 19 …… Acousto-optic modulator (acoustic optic deflector), 21 …… Output mirror, 23 …… Beam splitter, 25 …… Reflection grating, 27
… Fiber ring, 29… Taper coupler, 31… Dye laser pump.

 ──────────────────────────────────────────────────続 き Continued on front page (73) Patent holder 999999999 Leaky Lawrence UK Hampshire, S.O.2, 4P.J, Southampton, Bitterne Park, Oaktree Road 117 (73) Patent holder 999999999 Pure Simon Branchette Hampshire, S.O.2, 0 P.U., Southampton, Northern Union Road 18 (73) Patent holder 999999999 Paine David Nail Hampshire, S.U. -2,5N, Southampton, Bathreddon, Lawford, Ledroft Lane12 (72) Inventor Mias Robert Joseph Hampsey -, S.O.-2,4.G.B., Southampton, Bittane Park, Soold Road 34 (72) Inventor Leeky Lawrence Hampshire, S.K.・ O-2,4PJ, Southampton, Vittarne Park, Oakley Road 117 (72) Inventor Pure Simon Branchetet Hampshire, S-O-2, 0 P.U., Southampton, Northern Union Road 18 (72) Inventor Pain David's Nail Hampshire, S.O.-2, 5N.E.T., Southampton, Barsredon, Loeford, Let Draft Lane 12 (56) Reference JP-A-59-114883 (JP, A) JP-A-59-101629 (JP, A) JP-A-52-104942 (JP, A) , U) United States Huh 3729690 (US, A)

Claims (1)

(57) [Claims] 1. An active device comprising a core containing an active dopant of a rare earth transition element and a cladding, comprising an end-pumped glass optical waveguide, wherein a gain is obtained by stimulated emission of light. Is a communication silica glass fiber of a length, the fiber has a geometry for transmitting a single mode of light, the active dopant is an element selected from the group of erbium, terbium, and Light guide characterized in that the stimulated emission is provided by a three-level system that is contained and dispersed at a low concentration along the length of the fiber, the core having low propagation loss communication fiber characteristics. Discharge device.