WO2008115094A1

WO2008115094A1 - Optical comb-spectrum luminescent source

Info

Publication number: WO2008115094A1
Application number: PCT/RU2008/000084
Authority: WO
Inventors: Aleksandr Vasilievich Dmitruk; Vladimir Alekseevich Savostyanov
Original assignee: Federalnoe Gosudarstvennoe Unitarnoe Predpriyatie 'nauchno-Issledovatelsky I Tekhnologicheskiy Institut Optichestkogo Materialovedeniya Vserossiyskogo Nauchnogo Tsentra 'goi Im S.I. Vavilova'
Priority date: 2007-03-19
Filing date: 2008-02-07
Publication date: 2008-09-25
Also published as: RU2007111889A; RU2351046C2

Abstract

The inventive optical comb-spectrum luminescent source for photonic applications is designed in the form of a small-sized optical comb-spectrum source operating within a wavelength range of 1500-1600 nm for producing a spectral wavelength-division multiplexing effect in fiber-optic communications systems. The invention makes it possible to develop a small-sized and low-cost optical comb-spectrum source having a simple structural design. Comb-spectrum radiation is obtained by modulating the stimulated luminescence of rare-earth ions in a glass matrix by means of a Fabry-Perot resonator. The optical comb-spectrum luminescent source (1) comprises, located along the same optical axis, a pumping source (2), a focusing lens (3) and a light intensifier (4) in the form of an active solid-state cell (5) arranged inside the Fabry-Perot resonator. The active cell (5) is in the form of a plane-parallel luminescent plate made of a laser glass, the optical radiation axis of the pumping source being perpendicular to the plane of the Fabry-Perot resonator. The mirrors (6, 7) of the Fabry-Perot resonator can be applied directly to the planes of the plane-parallel luminescent plate. A highly-concentrated phosphate laser glass activated with ytterbium and erbium is used as an active medium.

Description

LUMINESCENT SOURCE OF THE OPTICAL

RADIATION WITH A COMBINED SPECTRUM.

The field of technology.

The invention relates to solid-state luminescent optical radiation sources and is intended for use in photonics as a compact optical radiation source with a comb spectrum in the wavelength range of 1500-1600 nm as a device for spectral densification in fiber-optic communication systems. The prior art.

It is known that for the implementation of spectral densification, it is necessary to use an optical radiation source with a comb spectrum described by the following formula:

S (f) = ∑ A _k δ (f- K Ω), - k <K <k (1) where S (f) is the amplitude of the signal at the frequency f, A _k is the amplitude of the k-th component of the spectrum, δ is the delta function, Ω is the modulation frequency, K, k are integers.

The prior art several designs of optical radiation sources with a comb spectrum. In the article by T. Kawapishi, T. Sakamoto, S. Shipada, M. Izutsu "Ortisal

Freixeps Comb Gepertos Uspg Ortil Fibeg Loors With Siplé-Sidébod Moduli (GEISE Elstgopis Exgess 2004, v. 1, p. 8, p. 217-221) described a source of optical radiation with a comb wavelength spectrum the sideband of the signal, consisting of four optical phase modulators, the frequency shift of which was exactly equal to the frequency shift of the radio frequency signal supplied to the modulator.The device consists of an optical modulator of the sideband of the signal, including electro-optical modulators lithium niobate with high-speed traveling wave electrodes, and a fiber-optic circuit, including an optical amplifier to compensate for losses when converting the signal in the modulator.The circuit has an input and output made in the form of an optical connector or a Bragg grating.

1 SUBSTITUTE SHEET (RULE 26) In the range 1550–1560 nm, a ridge spectrum was obtained with a distance between neighboring maxima of 0.08 nm. The disadvantage of this device is its complexity, the need to apply an electric signal with a frequency of the order of 10 GHz to the sideband modulator and the inability to achieve compactness due to the use of a fiber-optic circuit.

U.S. Patent JVb 9967981, published June 27. 2006 by class US 398 / 183_ and 398/186, describes a multi-wave communication device intended for use with a light source giving a beam of light at a wavelength λ. The device includes at least one optical modulator designed to convert a light beam into a modulated light signal, which consists of at least two additional spectral components, one at a wavelength of λ + Δλ, and the other at a wavelength of λ - Δλ. The device includes an optical splitting device for connecting with at least one optical modulator, for splitting the modulated optical signal into at least two such modulated light signals and for allowing one of the at least two additional spectral components. In US patent Jfc 5663822, published 02.09.1997 by CL. IPC

H04B 10/00 describes an optical comb spectrum generator using an optical white noise source. An optical comb spectrum generator generates radiation over a wide wavelength range using a white noise generator. The Fabry-Perot resonator filters the radiation according to wavelengths with the formation of a spectrum characterized by equidistant lines at resonant wavelengths X ₁ , λ ₂ X _n . The comb-spectrum generator is locked at a specific wavelength when comparing each carrier or average in carriers with an optical standard. The generated error signal is fed back to the comb spectrum generator. Resonances are split into N channels by means of a splitter. Each channel is modulated in a digital modulator. Channels are summed as an optical signal and amplified

2 SUBSTITUTE SHEET (RULE 26) to ensure proper power levels before being fed into the optical fiber.

In US patent Xs 4953166, published 08/28/1990 by CL. IPC H01SЗ / 0941, H01SЗ / 109, etc. describes a microchip laser in which an amplifying medium is located between two mirrors forming a resonator. The resonator length is selected so that the gain bandwidth of the amplifying medium is less than or equal to the distance between the resonator mode frequencies, so that the resonator mode frequency falls within the gain band. Nonlinear optical material is located inside or outside the resonator to generate new laser wavelengths. In another case, the microchip laser can be tuned, for example, thermally or by applying longitudinal or transverse voltage at the resonator frequency. The laser is optically pumped by a suitable source, such as a semiconductor injection laser or a laser ruler. Suitable amplification media include Nd: YAG, Nd: GSGG and neodymium pentaphosphate, and corresponding non-linear optical materials include MgO: LiNbO ₃ and KTP.

A common feature of all analogues is the complexity and high cost of devices that require precision assembly of components, and the inability to achieve a compact device.

The closest in design is the microchip laser according to US patent JVs 4953166. The disadvantage of the laser microchip is the inability to obtain a comb spectrum due to the selected ratio of the gain bandwidth of the amplifying medium and the distance between the resonator mode frequencies, as well as the use of expensive optical elements made of laser and nonlinear crystals.

Disclosure of the invention. The objective of the invention is to create a simple in design, compact, inexpensive to manufacture a source of optical radiation with a comb spectrum, capable of obtaining

3 SUBSTITUTE SHEET (RULE 26) radiation with a comb spectrum due to the use of the discovered effect - stimulated luminescence of rare-earth ions in a glass matrix modulated by a Fabry-Perot resonator. This solution allows you to use cheaper than laser crystals material - glass activated by rare-earth ions.

A new constructive solution to this problem is a luminescent source of optical radiation with a comb spectrum, which includes a pump source located on the same optical axis, a focusing lens, and an optical amplifier in the form of a solid-state active element located inside the Fabry-Perot resonator, in which the active element is plane-parallel a luminescent plate made of laser glass, and the optical axis of the radiation of the pump source is perpendicular to Fabry-Perot resonator bones. A feature of the new technical solution is the gain bandwidth of the amplifying medium, which significantly exceeds the distance between the frequencies of the modes of the Fabry-Perot resonator. Therefore, the number of stimulated emission modes emerging from the resonator can be large - 100 ÷ 200 in the case of an active medium based on glasses activated by rare-earth ions elements, since the number of modes is determined by the ratio of the gain bandwidth to the distance between the frequencies of the Fabry-Perot interferometer.

The reflecting elements of the Fabry-Perot resonator, which are mirrors, can be applied directly to the plane of the luminescent plate, which ensures the compactness of the device and simplifies its adjustment. The pump is pumped by a source with a power providing a gain mode, but not exceeding the generation threshold. At high levels of luminescence excitation, the primary luminescence quantum initiates the emission of a quantum at the same frequency. This effect is called superluminescence.

Obviously, the emission spectrum of superluminescence coincides with the gain spectrum of the medium. Superluminescence radiation, unlike

4 SUBSTITUTE SHEET (RULE 26) ordinary luminescence, is forced, the coherence length of which exceeds the length of the resonator. Therefore, at frequencies corresponding to the frequencies of the eigenmodes of the Fabry-Perot resonator, interference of superluminescence radiation is observed. As a result, the superluminescence spectrum of the amplifying medium, placed in the Fabry-Perot resonator, is modulated.

It is advisable to use laser glasses activated with Er, Yb, Nd, Tm, Cr, Yb / Er, Nd / Yb as a luminescent medium. The thickness of the plane-parallel luminescent plate is selected within 0.2-5.0 mm, which provides a distance between adjacent maxima of the comb spectrum from 400 to 16 GHz. At the indicated thicknesses of the resonator, it seems appropriate to use phosphate glasses with a high concentration of rare-earth elements (10-30 mass%) as an amplifying medium, which will ensure the efficient use of pump radiation.

A brief description of the drawings.

In FIG. 1 is a functional diagram of a device for observing a comb spectrum, where 1 is a luminescent optical radiation source with a comb spectrum, in which 2 is a pump source in the form of a semiconductor laser, 3 is a focusing lens, and 4 is an optical amplifier including an active element in the form of plane-parallel glass a plate 5 placed inside a Fabry-Perot resonator, mirrors 6 and 7 of which are deposited on the plane of the plate 5; 8 and 9 — aperture diaphragms, 10 — a light filter that cuts off pump radiation, 11 — a fiber optic cable, 12 — a monochromator, 13 — a photodetector, 14 — a recording device.

In FIG. 2 shows an optical amplifier 4, which is a plane-parallel plate 5 in the form of a disk made of laser glass with dielectric coatings deposited on its plane-parallel surfaces - mirrors 6 and 7, which reflect pump radiation and are transparent at the generation wavelength.

5 SUBSTITUTE SHEET (RULE 26) In FIG. 3 shows the luminescence spectrum of phosphate glass activated by ytterbium and erbium, which is the most suitable material for the manufacture of a luminescent plate.

The use of this plate with a thickness of 1.0 mm showed the possibility of obtaining a comb spectrum due to modulation of the erbium luminescence spectrum with a frequency of Ω = 80 GHz.

The modulated spectrum is shown in FIG. 4. The frequency of 80 GHz corresponds to the difference in wavelengths Δλ = 0.75 nm between adjacent maxima with wavelengths λ _m ÷ i and λm in the comb luminescence spectrum shown in FIG. 5. When registering luminescence in the direction perpendicular to the axis of the Fabry-Perot interferometer, this effect is not observed.

The frequencies of the eigenmodes of the Fabry-Perot resonator, v _m , at which the transmission of the resonator reaches a maximum: V _m = m ^• ΔVсlс, ΔVсafc = 1 / (2 ^* L ^• П), ΔVсlс = Δλсlс / λ ² (2) where v _m = 1 / λш, m = 1,2,3 ... L is the thickness of the resonator, n is the refractive index.

Substitution of the experimental values L = 1.0 mm, n = 1.54, λ = 1.535 nm in (2) gives Δλ ^ i _с = 0.75 nm, which coincides with the experimental value obtained from the luminescence spectra. Consequently, modulation of the erbium luminescence spectrum by the transmission spectrum of the Fabry-Perot resonator was experimentally obtained.

The effect obtained is stimulated emission, the coherence length of which exceeds the length of the Fabry-Perot resonator, and the spectrum coincides or is close to the luminescence spectrum of erbium, i.e. superluminescence achievable at high levels of luminescence excitation, when the primary luminescence quantum initiates the emission of another quantum at the same frequency.

Independent confirmation that the modulated part of the radiation emerging from the luminescent plate is superluminescence of erbium is the exponential

6 SUBSTITUTE SHEET (RULE 26) dependence of the intensity of this radiation on the intensity of the pump radiation.

The main advantages of the proposed comb-type fluorescent light source over the well-known technical solutions are: ultra-small dimensions of the active medium (less than 1 mm ³ ), simplicity of design (plane-parallel plate with mirrors deposited on its surface), the possibility of organizing the simultaneous production of a large number of microarrays (from a plate with a diameter 30 mm can produce up to 200 microchips of the same quality). The best embodiment of the invention.

Laser phosphate glass containing 6.5 mol% YbgO3 and 0.5 mol% ErgOz was used as the material of the active element 5 — an amplifying medium. A plane-parallel plate with a diameter of 30 mm and a thickness of 1 mm was made of glass. The plane parallelism of the wafer was no worse than 15 ". On the polished planes of the wafer-disk, multilayer dielectric mirrors 6 and 7 were applied with reflection coefficients of 99.9 and 97% at a wavelength of 1535 nm, transparent at a wavelength of exciting light of 965 nm. For luminescence excitation was used pump source 2 is a semiconductor laser with a luminous body 1 x 100 μm in size, with an output power of up to 1 W at a generation wavelength of 965 nm. a single plate — the active element 5 — into a spot with a diameter of 100 μm. Registration of the radiation output from the output surface of the plate 5 was carried out in the direction coinciding with the direction of the exciting radiation. The detected radiation passed through two aperture diaphragms 8 and 9, which recorded the signal with an angular aperture Г To prevent the scattered light pump source 2 from reaching the photodetector 13, a cut-off filter 10 made of IKC-5 glass 1 cm thick was placed in front of the aperture diaphragm 9. nd the radiation passing through the diaphragm 8 and 9 are incident on the entrance aperture of the fiber optic cable 11 January mm diameter and

7 SUBSTITUTE SHEET (RULE 26) 1 m long made of quartz glass. Using an optical cable 11, the radiation was transmitted to the input slit of the monochromator 12. As a radiation photodetector 13, a photodiode was used placed on the output slit of the monochromator 12. The resulting comb spectra of the radiation of the luminescent light source were reflected on the screen of the monitor-recorder 14 (see Fig. 5) . The maximum spectral resolution of the setup was OD nm.

Industrial applicability. Control experiments showed that the indicated spectral resolution of the setup is sufficient to record the fine structure of the spectra and measure the modulation depth, but not enough to measure the half-width of the fine-structure lines, which is less than OD nm. The depth of spectral modulation at a given wavelength M (X _k ) was determined using equation (3): M (X _k ) = [J _1118x (X _k ) - J _1H i _n (^ _{+ 1} -)] / Jm _3x (Xk ) (3)

Where J (X) is the luminescence intensity, Xk, Xk + 1 are adjacent wavelengths at which the maximum and minimum of interference are observed. The maximum achieved value of the modulation depth for radiation with a wavelength of 1533.8 nm is 90%. It has been established that for radiation with wavelengths in the range 1540-1560 nm, variations in the modulation depth are insignificant. It follows that it is possible to create a 30-channel optical radiation source in the specified spectral range with the same radiation intensity in each spectral channel.

8 SUBSTITUTE SHEET (RULE 26)

Claims

CLAIM.

1. Luminescent optical radiation source with a comb spectrum, including a pump source located on the same optical axis, a focusing lens and an optical amplifier in the form of a solid-state active element located inside the Fabry-Perot resonator, characterized in that the active element is a plane-parallel luminescent plate made from laser glass, and the optical axis of the radiation of the pump source is perpendicular to the plane of the Fabry-Perot resonator.

2. A luminescent optical radiation source according to claim 1, characterized in that the Fabry-Perot resonator mirrors are applied directly to the plane of the luminescent plate.

3. A luminescent optical radiation source according to claim 1 or claim 2, characterized in that a pump source with an intensity capable of providing a positive gain of the luminescent medium, but less than the generation threshold, is used.

4. The luminescent optical radiation source according to claim 1 or claim 2, characterized in that laser glasses activated by Er, Yb, Nd, Tm, Yb / Eg, Nd / Yb are used as the luminescent medium.

5. The luminescent optical radiation source according to claim 1 or claim 2, characterized in that the plane-parallel luminescent plate has a thickness in the range of 0.2-5.0 mm.

6. A luminescent optical radiation source according to claim 1 or claim 2, characterized in that a 1.0 mm thick phosphate glass plate activated by ytterbium and erbium is used as a plane-parallel luminescent plate.

9 SUBSTITUTE SHEET (RULE 26)