RU2411621C1 - Quantum amplifier - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: quantum amplifier consists of laser media activated by neodymium and ytterbium, and optical pumping apparatus. The laser medium consists of at least two active elements, one activated by ytterbium and the other by neodymium. The amplification spectra of the active elements are frequency shifted relative each other, and the optical pumping apparatus are configured to selectively pump in absorption bands of ytterbium and neodymium at different wavelengths using semiconductor lasers or light-emitting diodes.

EFFECT: possibility of amplification with amplification spectrum width greater than the spectrum width of media activated by ytterbium and neodymium.

5 cl, 8 dwg

Description

The invention relates to solid-state quantum amplifiers and can be used to create powerful pulsed laser systems with subpicosecond and femtosecond (10 ^-13 -10 ^-15 sec) laser pulse duration.

The implementation of laser systems with subpicosecond and femtosecond laser pulses makes additional demands on the spectral bandwidth of amplification of quantum amplifiers, which are absent when amplifying longer pulses, for example, nanosecond ones.

It is known that the duration of a laser pulse - t _p is related to the width of its spectrum - Δv by the ratio:

The quantitative ratio between the pulse duration and the half-width of its spectrum depends on the shape of the pulse.

For Gaussian pulses, the dependence of the intensity on time is described by a function of the form:

where P (t) is the envelope shape of the pulse, P _p is the peak value of the amplitude, t _p is the pulse duration at half the height of the pulse, t is time.

For pulses, the shape of which is described by expression (2), relation (1) can be represented as:

Using expression (3), it is possible to calculate the half-width of the spectrum of a laser pulse depending on its duration at a given wavelength of laser radiation. The calculation results for a wavelength of 1 μm are shown in figure 1.

As can be seen from Fig. 1, a Gaussian pulse with a duration of 100 Fs has a half width of 10 nm, while the typical half width of the gain spectrum of the most common neodymium laser media in practice is 20–30 nm.

When a laser pulse propagates through an amplifying medium, the width of the gain spectrum of which is comparable to the width of the spectrum of a laser pulse, the spectrum of the amplified pulse narrows and, as a result, its duration increases due to relation (1), which affects the output power of the pulse. The physical reason for this effect is that the highest gain is provided for the maximum of the pulse spectrum, and the spectral components on the "tails" of the pulse are amplified to a lesser extent.

In the article by A.A. Andreev, A.A. Mack, V.E. Yashin. Generation and application of superstrong laser fields (Review). (“Quantum Electronics”, v.24, No. 2, 1997, pp. 99-114), the influence of the gain spectrum width on the propagation of an ultrashort laser pulse is numerically analyzed and it is shown that in the case of amplifiers on phosphate neodymium glass having a half-width of the gain spectrum of about 20 nm, with a total gain of 10 ^{9, the} spectrum of a pulse with a duration of 100 Fs narrows by 4–5 times, and the duration of a compressed pulse increases by the same amount.

In connection with the foregoing, the search for laser media having both the largest width of the gain spectrum and the lifetime of the excited state for the efficient accumulation of pumping energy of exciting light, as well as new ways of amplifying ultrashort laser pulses, is of particular relevance.

In an article by John Nees, Subrat Biswal, Frederic Druon, Jerome Faure, Mark Nantel, Gerard A. Mourou, Akihiko Nishimura, Hiroshi Takuma, Jiro Itatani, Jean-Christophe Chanteloup and Clemens Honninger. Ensuring Compactness, Reliability and Scalability for the Next Generation of High-Field Lasers (Invited Paper). (IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO.2, MARCH / APRIL 1998, p.376-384) analyzed the extreme capabilities of various laser media in terms of obtaining the maximum achievable values of peak power and minimum values of laser pulse duration .

From this analysis it follows that for laser glasses (silicate, phosphate) activated by neodymium, the theoretical minimum values of the laser pulse can reach 60 ÷ 70 Fs, and the maximum peak power is 100 terawatt / cm ² .

For ytterbium-activated glasses, the minimum laser pulse duration can be 15–20 Fs, and the maximum peak power is about 1 petawatt / cm ² .

A detailed study of the possibilities of ytterbium-activated glasses for the generation and amplification of ultrashort pulses was carried out by C. Hönninger, R. Paschotta, M. Graf, F. Morier-Genoud, G. Zhang, M. Moser, S. Biswal, J. Nees , A. Braun, GAMourou, I. Johannsen, A. Giesen, W. Seeber, U. Keller. "Ultrafast ytterbium-doped bulk lasers and laser amplifiers (Invited Paper)." (Appl. Phys. B 69, pp. 3-17 (1999)). The authors of the study analyzed the advantages of laser materials activated by ytterbium, compared with materials activated by neodymium, in terms of obtaining laser pulses of high power and minimum duration. It is shown in the paper that the width of the ytterbium luminescence spectra is significantly greater than the width of the neodymium luminescence spectra, which allows a significant reduction in the duration of laser pulses. For ytterbium-activated glasses, the possibility of generating laser pulses of 60 fs duration was experimentally demonstrated.

In US patent No. 5235606, published 10.08.1993, according to CL. US 372/25, 372/69 and 372/20 describe a laser system for amplifying ultrashort pulses based on neodymium-activated laser glasses. To increase the gain bandwidth of a quantum amplifier, it is proposed in the patent to use a composite amplifier consisting of various types of laser glasses activated by neodymium, whose gain spectra are shifted relative to each other in frequency. As a result of the frequency shift of the gain spectra of the individual elements of the composite amplifier, the width of the total gain spectrum exceeds the width of the gain spectrum of the individual components. The disadvantage of this device is that the width of the gain spectrum of a composite element based on neodymium-activated glasses remains substantially less than the width of the gain spectrum of ytterbium activated glasses.

In US patent No. 5956354, published 09/21/1999, according to CL. US 372/18, 372/23, 372/68, 372/94, 372/69 describes a laser system consisting of a master oscillator based on crystals activated by neodymium and an amplifier based on glasses activated by neodymium. Crystals and glasses are selected in such a way that the luminescence spectra of the neodymium activator in them overlap as much as possible. The following pairs of laser materials meet this condition: Nd: YLF crystals and Nd-phosphate glass, as well as Nd: YAG and Nd-silicate glass. The disadvantage of this device is that the minimum duration of the laser pulse is determined by the gain bandwidth of the master oscillator - the laser crystal. For a Nd: YLF crystal, the half-width of the gain spectrum is 6 cm ⁻¹ (0.7 nm), and for a Nd: YAG crystal, it is 12 cm ⁻¹ (1.4 nm), respectively. Narrow gain spectra of the master oscillator make it possible to obtain laser pulses of about 2 ps duration.

In US patent No. 6212215, published April 3, 2001, according to CL. USA 372/68, 372/41, 372/6, M.C. H01S 3/14 describes a hybrid laser system consisting of a master oscillator based on neodymium-activated laser media and a power amplifier using ytterbium-activated crystals or glasses as an active medium. A distinctive feature of this approach is that the maxima of the gain spectra of neodymium and ytterbium coincide in wavelengths. The disadvantage of the hybrid laser system under consideration is that the minimum laser pulse duration is determined by the gain spectrum width of the neodymium-based master oscillator, which is substantially less than the gain spectrum width of ytterbium-activated media.

A common feature of all analogues is that the minimum laser pulse duration is determined by the gain spectrum width of active media based on neodymium, which is significantly less than the gain spectrum width of ytterbium-activated media, which does not allow the generation / amplification of laser pulses with a duration of less than 100 Fs.

Closest to the proposed method for amplifying ultrashort laser pulses is a hybrid laser system according to US patent No. 6212215, the disadvantage of which is the narrow gain spectrum of a master oscillator based on media activated by neodymium.

The objective of the invention is to develop a quantum amplifier with a gain spectrum width exceeding the spectrum width of media activated by both ytterbium and neodymium.

To achieve this goal, a quantum amplifier is proposed, consisting of laser media activated by neodymium and ytterbium, and optical pumping means, the laser medium being made of at least two active elements, one of which is activated by ytterbium and the other by neodymium, and the gain spectra of the active elements are shifted relative to each other in frequency, in addition, the optical pumping means are configured to pump selectively into the absorption bands of ytterbium and neodymium at different wavelengths with oschyu semiconductor lasers or LEDs.

Since the absorption bands of neodymium and ytterbium do not overlap, the pump radiation of the active element from glass activated by neodymium is not absorbed by the active element from ytterbium activated glass, nor is the pump radiation of the active element from glass ytterbium activated by the active element made of neodymium activated glass. Therefore, the active elements can be arranged in any order.

The gain spectrum of a Yb-Nd quantum amplifier is the sum of the gain spectra of the individual active elements: the active element from glass activated by ytterbium and the active element from glass activated by neodymium.

The essence of the claimed invention is illustrated by drawings, where

figure 1 shows the dependence of the half-width of the spectrum of a laser pulse of a Gaussian shape on its duration;

figure 2 shows a functional diagram of a quantum amplifier;

figure 3 presents a schematic representation of the gain spectra of an active element of glass activated by ytterbium, neodymium, and the gain spectrum of a Yb-Nd quantum amplifier;

figure 4 presents a schematic representation of the gain spectra of the active element of phosphate glass activated by ytterbium 1, neodymium-2, the gain spectrum of the active element of silicate glass activated by neodymium-3, and the gain spectrum of the Yb-Nd quantum amplifier - 4;

figure 5 shows the absorption spectra and luminescence of Yb ³⁺ in silver-containing phosphate glasses activated by ytterbium;

figure 6 shows the gain spectra of ytterbium in phosphate glasses with different degrees of inversion n ₂ , where n ₂ is the fraction of ytterbium ions in the excited state Yb ³⁺ ( ² F _5/2 ), 1 - n ₂ = 0.2, 2 - n ₂ = 0.4, 3 - n ₂ = 0.55, 4 - n ₂ = 0.6;

7 shows the gain spectra of glasses activated by ytterbium - 1, neodymium - 2 in phosphate glasses of the same composition. The degree of inversion is n ₂ in ytterbium-activated glasses, n ₂ = 0.2;

on Fig shows the gain spectra of glasses activated by ytterbium - 1, neodymium - 2 and Yb-Nd of the quantum amplifier - 3, 4 at different pump levels of the active element from glass activated by neodymium: G (Nd) = 0.5G (Yb) - 3, G (Nd) = G (Yb) - 4, where G is the gain at the maximum of the activator band. The degree of inversion is n ₂ in ytterbium-activated glasses, n ₂ = 0.2.

The functional diagram of a quantum amplifier is shown in Fig. 2, where 1, 2 are semiconductor pump lasers with a radiation wavelength corresponding to the absorption bands of neodymium and ytterbium, respectively, 3 are micro-lenses, 4 is a fiber light guide, 5 is an active element made of ytterbium activated glass, 6 - active element of glass activated by neodymium.

Figure 3 schematically shows the gain spectrum of the active element from glass activated by ytterbium-1, the gain spectrum of the active element from glass activated by neodymium-2, and the gain spectrum of the Yb-Nd quantum amplifier - 3.

In contrast to the prototype, the gain spectrum of the active element of glass activated by ytterbium is shifted in frequency (wavelength) relative to the gain spectrum of the active element of glass activated by neodymium. Therefore, the gain spectrum of the Yb-Nd quantum amplifier is wider than the gain spectra of individual active elements made of ytterbium-activated glass and neodymium-activated glass.

A further development of the proposed approach may be the use of not only two, but three or more amplifying elements in a Yb-Nd quantum amplifier, one of which is made of ytterbium activated glass, and the rest of neodymium-activated glasses of various compositions. In this case, the gain spectra of neodymium-activated glasses should be shifted relative to each other in frequency. This situation is realized, in particular, for phosphate and silicate laser glasses activated by neodymium, for which the maximum gain bands correspond to wavelengths of 1054 nm and 1060 nm, respectively. An illustration of the foregoing is FIG. 4, which schematically shows the gain spectra of active elements made of ytterbium and neodymium activated glasses, as well as the gain spectrum of a quantum amplifier consisting of three active elements.

One of the possible ways of implementing the present invention may be a quantum amplifier consisting of active elements in the form of disks (plates) made of laser glasses activated by ytterbium and neodymium. The thickness of the disk, as well as the concentration of the activator are determined by the specific design features of the laser system and the selected pumping method: longitudinal, transverse, waveguide. A possible optical design of such an amplifier is shown in FIG. 2.

A Yb-Nd quantum amplifier can be constructed not as separate amplifying elements, but as a monoblock when the amplifying elements in the form of a plate of glasses activated by ytterbium and neodymium are connected to each other, for example, by sintering glasses. This approach simplifies the design of a quantum amplifier, facilitates the manufacture of high-precision optical elements and their subsequent adjustment in the manufacture of a quantum amplifier.

A concrete implementation example is the results of numerical simulation of the gain spectra of active elements made of phosphate glasses activated by ytterbium and neodymium, obtained by us on the basis of an experimental study of the absorption and luminescence spectra of these glasses.

In the case of ytterbium glasses, where the generation is realized according to a 3-level scheme, the gain spectrum is very different from the luminescence spectrum due to the strong reabsorption of luminescence. This conclusion follows from the experimental data shown in figure 5. Figure 5 presents the absorption spectra of 1 and luminescence of 2 phosphate glass activated by ytterbium. Ytterbium luminescence was excited by radiation from a semiconductor laser with a wavelength of 965 nm.

For a correct analysis of the width of the gain spectrum, it is necessary to calculate it on the basis of experimentally obtained absorption cross sections - σ _abs and emission - σ _emis , whose spectrum coincides with the absorption and luminescence spectra. The absolute values of the absorption and luminescence cross sections at the maximum of the bands are calculated and are equal for the studied glasses σ _abs = 1.1 · 10 ^-20 cm ² , σ _emiss = 1.16 · 10 ^-20 cm ² .

The absorption and emission cross sections determined from the experimental data make it possible to calculate the gain spectrum (dependence of the gain on the wavelength) of the ytterbium-activated glass reinforcing element.

The gain of the active medium - g is defined as:

where L _g is the length of the active medium, N _tot is the volume concentration of the activator - Yb ³⁺ , n ₂ is the fraction of the activator in the excited state Yb ³⁺ ( ² F _5/2 ), n ₁ is the fraction of the activator in the ground state of Yb ³⁺ ( ² F _7/2 ), n ₂ + n ₁ = 1.

The volume concentration of the activator - ytterbium in the studied glasses is N _tot = 1.7 · 10 ²¹ cm ^-3 .

The amplification spectra of ytterbium in the studied glasses for different levels of inversion - n ₂ are shown in Fig.6. The degree of inversion is proportional to the pump power.

As follows from the data shown in Fig.6, the gain spectra of ytterbium are very different from the luminescence spectra - Fig.5 and broaden with increasing degree of inversion - n ₂ . At relatively low pump levels (n ₂ = 0.2), the half-width of the gain spectrum is about 50 nm.

In the case of neodymium-activated glasses, for the laser transition

Nd ^{3+ (4} F _3/2) → Nd ^{3+ (4} / _11.02)

a 4-level generation scheme is implemented, for which the gain is described by expression (5):

where L _g is the length of the active medium, N _tot is the volume concentration of the activator neodymium, n ₂ is the fraction of the activator in the excited state Nd ³⁺ ( ⁴ F _3/2 ).

It follows from (5) that the shape of the gain spectrum of neodymium glasses, unlike glasses activated by ytterbium, coincides with the shape of the luminescence spectrum of neodymium and does not depend on the degree of inversion, i.e., on the pump intensity.

The neodymium gain spectrum in the studied glasses is shown in Fig. 7, where 1 is the ytterbium gain spectrum with an inversion degree n ₂ = 0.2 and 2 is the neodymium gain spectrum in glasses of the same composition. The amplifying element made of neodymium-activated glass was pumped by radiation from a semiconductor laser with a wavelength of 800 nm, the volume concentration of activator ions was 5 · 10 ²⁰ cm ^-3 .

As follows from the data shown in Fig. 7, the maximum of the neodymium gain spectrum is shifted relative to the ytterbium gain spectrum by approximately 30 nm to the long-wavelength region of the spectrum, and the half-width of the ytterbium gain spectrum is approximately two times larger than in neodymium glasses and is about 50 nm .

A shift in the gain spectra of ytterbium and neodymium in the glasses under study makes it possible to realize a gain spectrum in the Yb-Nd amplifier element whose half width is greater than the half width of the gain spectra of the individual components of the amplifier.

For a quantum amplifier consisting of ytterbium and neodymium active elements, the gain spectrum is the sum of the gain spectra of the constituent elements, each of which independently amplifies the input laser pulse in its gain band. Since the optical pumping of ytterbium and neodymium amplifier elements is carried out independently at different wavelengths - 965 nm and 800 nm, respectively, the shape of the gain spectrum of the Yb-Nd quantum amplifier can be varied over a wide range by changing the pump power (degree of inversion) of ytterbium or neodymium glass.

An example of the implementation of the invention (amplification of ultrashort laser pulses) are the results of modeling the gain spectra of a Yb-Nd quantum amplifier obtained from the gain spectra of ytterbium and neodymium in phosphate glasses of the same composition. These data are presented in FIG.

When the neodymium pump is turned off, the gain spectrum of the Yb-Nd quantum amplifier coincides with the gain spectrum of the ytterbium-activated glass active element with a half width of 49.4 nm. When ytterbium pump is switched off, the gain spectrum of the Yb-Nd quantum amplifier coincides with the gain spectrum of the active element made of neodymium-activated glass, whose maximum is shifted relative to the maximum of the ytterbium spectrum by 29.4 nm and has a half width of 24.4 nm. With simultaneous pumping of ytterbium and neodymium, when the gain of the active element from glass activated by neodymium is equal to half the gain of the active element from glass activated by ytterbium, the integrated gain spectrum broadens and its half-width is 55.7 nm.

The data presented are experimental evidence that the spectrum width of the Yb-Nd quantum amplifier exceeds the width of the gain spectrum of active elements made from neodymium-activated glasses and active elements made from ytterbium-activated glasses. Thus, the proposed device solves the problem: the development of a quantum amplifier with a gain spectrum width exceeding the spectrum width of media activated by both ytterbium and neodymium

Claims (5)

1. A quantum amplifier consisting of laser media activated by neodymium and ytterbium, and optical pumping means, characterized in that the laser medium is made of at least two active elements, one of which is activated by ytterbium, and the other by neodymium, wherein the gain spectra active elements are shifted relative to each other in frequency, and optical pumping means are capable of pumping selectively into the absorption bands of ytterbium and neodymium at different wavelengths using semiconductor lasers or LEDs. 2. The amplifier according to claim 1, characterized in that the laser medium is made in the form of a glass monoblock, in which the glasses activated by ytterbium and neodymium are spatially spaced. 3. The amplifier according to claim 2, characterized in that the glass monoblock is made by sintering of glasses activated by ytterbium and neodymium. 4. The amplifier according to claim 1, characterized in that several active elements are used in the laser medium, one of which is activated by ytterbium, and the remaining elements of various compositions are activated by neodymium, the gain spectra of which are shifted relative to each other in frequency. 5. The amplifier according to claim 4, characterized in that phosphate and silicate glasses are used as active elements activated by neodymium.

Images