JPS5824030B2 - Laser that can tune a wide range of wavelengths - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to lasers, and more particularly to widely tunable chromium-doped aluminum beryllate lasers and the operation of these lasers.

Laser materials depend on the ability of these materials to absorb energy in such a way that many atoms (or molecules) are excited to a higher energy level than their ground state, allowing the material to undergo stimulated emission. There is.

If light of a suitable wavelength is irradiated onto such an excited material, the material can stimulate further emission of light having the same wavelength, phase and direction as the irradiated light.

Such light exhibits optical amplification by increasing the intensity of the incident light.

Laser action was developed in 1960 by ruby (Al2O3:Cr).
”), was demonstrated for the first time using a crystalline solid system.

Since then, this lasing action has been achieved in gas, liquid and glass systems as well as in other crystalline solid systems.

All of these lasers are wavelength tunable, although most are only tunable over a sub-nanometer bandwidth.

Lasers whose wavelengths can be tuned over a wide range (i.e., lasers with a wide tunable range) are used in science,
Although extremely useful in industrial applications (e.g. isotope separation), these lasers are also very useful in liquid medium dye lasers (e.g. isotope separation).
ers) were originally restricted.

However, [phonon-
ter m1nated ) or [electronic vibration J(v
There are only a few known types of widely tunable solid-state host-type lasers called ib-ronic lasers.

The most commonly widely tunable dye lasers have several drawbacks.

Are these lasers common? 2. Average power performance is generally limited because these liquid hosts are thermally unstable and high power applications require auxiliary equipment to maintain dye flow.

The lifetime of these lasers is limited by the decomposition of the organic dye material.

Some of the dyes and solvents are toxic and/or corrosive.

Furthermore, dye lasers are inherently incapable of Q-switching.

A host type laser uses impurity ions added to a solid host at a dilute concentration as a laser component.

There are various host type lasers in electronic vibration lasers.
They derive their tunability from the emission of vibrational quanta (acoustic quanta) that occurs simultaneously with the emission of photons (optical quanta).

The energy of optical and acoustic quanta emitted simultaneously by an electron vibration laser becomes the energy of a purely electronic, ``non-photon'' transition state.

The wide wavelength tuning capability of electro-oscillatory lasers is due to the wide energy acoustic quantum continuum that complements the emission of photons.

Regarding some electron vibration lasers, L-F-Johnson et al.
Basis optical maser J (Phonon-Ter-min
ated 0ptical Masers Puff Physical Review (Phys-Rev・) 149.179
.

Described in 1966.

These all lead to critical tunability discontinuities associated with the structure of these electronic vibrational spectra.

Furthermore, these lasers must operate at low temperatures.

Electron vibration laser emission at room temperature was observed in the infrared at 2.17 μm in BaY2F8 containing Ho3+ as an impurity [(L.F. Johnson)
-Johnson) and H.J. Guggenheim (IBaY2)
"Electron- and Photon-Terminated Laser Emission from Ho"+ of Fa"
n Terminated Laser Emissi
on fromHo”+1nBaY2F8J (IEE
E J -Quantum Electron, )
)oL However, the pulse darkness value of this laser is quite high (450 J).

Electron vibration laser emission at room temperature with vacuum ultraviolet rays is Nd3+,
Predicted in (doped) trifluorides containing Er3+-1 and Tm3+ [Nd''-1Er5+ by K, H, Yang and JA, DeLuca]
-1 and Tm''-containing (doped) trifluorides and tunable coherence sources from 1650-2600, Appl, Phys, Lett.

29.499.1c+76].

However, the expected tunable laser emission from such materials requires laser pumping due to the short lifetime of the fluorescence.

Tunable laser action has also been predicted for broadband room-temperature solid-state lasers operated with visible light (R.W. Boyd, R.W.).

Boyd) and G.F. Owen (J.F.

Owen) and K.G.

J, Teegarden) [Laser action of the em center of lithium fluoride (La5er Ac
tion of M Centers in Li
thiumFluoride) J IEEE Quantum Electron (IEEE J,
QUIE-1 of Quantum Electron
4 (QE-14), 697, 1978).

Tuning the output wavelength of a tunable laser can be achieved by incorporating any optical element having wavelength-selective properties within the optical resonator.

As a result, it becomes easy to amplify only light having a certain wavelength by the active medium.

The only constraints on the selection of this element are:

That is, the selection of this element should not introduce optical losses of this magnitude at the wavelength chosen to prevent laser oscillations.

One example of such an element is a prism placed within an optical cavity between the laser medium and a mirror at one end.

This prism reflects light of different wavelengths at different angles.

To maintain laser resonance between the end mirrors forming the optical cavity, the mirrors must be perfectly aligned so that the reflected light is perpendicular to these surfaces. .

Thus, by rotating one of these optical cavity mirrors relative to the light passing through the prism, one confines the laser resonance to a limited, selectable portion of the emission spectrum. Is possible.

Another method of pacing utilizes, instead of prisms, birefringent filters consisting of adjustable optical gratings or one or several birefringent plates that can be pivoted or rotated to select the output wavelength. do.

Tuning can also be accomplished by placing an elongated chromatic lens at the exit end of the laser.

The lens is moved closer or further away from the laser medium to change the wavelength of the laser emission.

The spectral width of the emitted radiation can be controlled over a wide range by appropriately selecting the tunable elements built into the resonator.

By using elements with a continuously narrow transmission bandwidth, for example a multiple etalon, the output power of the laser can be narrowed down to a single oscillation mode of the laser's resonator.

Bukin et al. (Sov, J, Q
uan tum Electron, 8(5), 671
, May 1979) reported stimulated emission from alexandrite in 77.

The output wavelength can be varied over a range of 0.5 nm or less by changing the crystal temperature.

On December 14, 1976, R. C. Morris (R.
, C. Morris) and C.F. Klein (C.F.
, U.S. Patent No. 3.997 issued to C11ne),
No. 853 discloses 3 oriented substantially along the α-C plane.
IJ containing (doped) chromium as an impurity at a concentration of about 0.005 to 1.0 atomic percent
6804A (680
, 4 nm) is disclosed.

These crystals have a chrysoberyl structure, which is isomorphic with an orthorhombic structure (exclusite).

The space group of this structure is α=0.9404 nm, b=0
．． pnma with lattice parameters of 5476 nm and c=0.4427 nm.

As used in this specification, the term alexandrite refers to beryllium aluminate (BeA 1204) containing trivalent chromium with a chrysoberyl structure as an impurity.
: Cr + a).

According to the present invention, a broadly wavelength tunable laser is provided with a laser medium consisting of a single crystal of alexandrite having a concentration of trivalent chromium doping of about 0.002 to 10 atomic percent with respect to the aluminum sites; a device for guiding said laser medium emitting radiation; and pacing means for pacing said coherent radiation in the red to infrared spectral range.

It was quite unexpected that the laser medium of this invention is broadly wavelength tunable.

The R-line fluorescence of the laser medium with peak power at 680.4 nm (FIG. 1) is similar to that of ruby.

Note that ruby laser emission is not thought to be able to be tuned over a wide range of wavelengths.

By analogy, the laser emission disclosed in US Pat. No. 3,997,853 at 680.4 nm would not be expected to tune similarly broadly.

Furthermore, the above paper by Bukin et al.
Not only do they report emissions at the line (at liquid nitrogen temperature), but also at 679.87-680.331 m.
% or less than 0.5 parts m are also specifically mentioned.

This indicates that the laser cannot be broadly tuned.

Indeed, after discovering that the room temperature laser emission wavelength (using moderate excitation energies) is considerably longer than 680 parts m, and realizing that this emission lies within the vibrational sideband of a purely electronic transition, The potential of alexandrite as a broadly tunable laser medium was investigated and established.

Besides tunability, another advantage offered by alexandrite laser media is four-level operation.

Four-level lasers are a type of laser characterized by the fact that the final level of laser transmission is not the ionic ground state.

In such lasers, very little, if any, reabsorption of the laser emission by unexcited ground state ions occurs within the laser host.

This allows for low thresholds and high efficiency operation of the laser.

In alexandrite, as in other acoustic quantum-final lasers, the transmission of the laser involves the simultaneous emission of acoustic and optical quanta, so that the reabsorption of this emission involves the simultaneous absorption of both quanta. required.

The possibility of this reabsorption is low, and therefore the observed threshold of lasing in alexandrite 4-level lasers is low.

The tunable laser of the present invention overcomes the deficiencies of both tunable dye lasers and previously known electron vibration lasers.

High power operation is achieved without the need for auxiliary equipment;
Q-switch operation is possible; and the laser medium is stable and non-toxic and non-corrosive.

The emission is partly in the visible region of the spectrum.

Using the laser of the present invention to generate tunable coherent radiation, it has been unexpectedly discovered that the gain of the laser increases with increasing temperature.

Therefore, it is preferable to operate at high temperatures.

However, if desired, ambient or near ambient temperatures can be used.

The present invention will be explained in more detail below.

The laser medium used in the present invention contains oxidized chromium impurities at a concentration of about 0.002 to 10 atomic percent, preferably about 0.601 to 1.0 atomic percent, more preferably about 0.1 to 0.4 atomic percent, with respect to aluminum sites. Consists of a single crystal of alexandrite with (doping).

This laser medium is an optical pump source (opt).
ical pumping 5sources) and emits coherent radiation.

Preferably, the radiation is directed along the a-c plane of the crystal and has a polarized electric vector substantially perpendicular to the a-c plane.

A detailed description of the fabrication and properties of this laser medium is disclosed in US Pat. No. 3,997,853.

The laser medium of this invention may be pumped using any suitable optical pumping source.

In this case, it may be non-coherent or coherent, continuous or pulsed.

Sufficient excitation is particularly achieved using wavelengths shorter than about 700 nm.

Examples of suitable incoherent sources are gaseous emission lamps filled with xenon and/or krypton (designed for continuous or pulsed operation) and mercury, sodium, cesium, rubidium and/or potassium. It is a metal vapor source such as

Continuous mercury arc lamps are particularly suitable as pumping sources for continuous laser operation, and pulsed xenon arc lamps are also particularly suitable as pumping sources for pulsed laser operation.

The coherent pumping source must have an emission wavelength that is absorbed by the Cr''+ dopant ion ground state, but not excessively absorbed by the excited state.

For continuous excitation, krypton ion and argon ion lasers are representative.

For pulsed laser pumping, almost any coherent light source with sufficient power and an emission wavelength of 595 parts m or less is an effective pump for current lasers.

Examples of suitable light sources are two stacked Nd:YAG1 excimer lasers and a nitrogen laser.

Any conventional tuning means can be used to tune the laser of the present invention.

Examples of suitable tuning means include prisms, optical gratings, birefringent filters, multilayer dielectric coated filters, or lenses with longitudinal chromatic aberration.

A particularly suitable one is G. Holtom.
) and Te5chke (0, Te5chke), “Design of birefringent filters for high-power dye lasers” (Desig
n of a Birefringent F
filter for High-Power Dye
La5ers), IEEE Quantum Electron (IEEE J, Quantum
Electron), QE-10, 577, 1974
This is a general type of birefringence filter described in 2010.

This type of filter is also called a Lyot filter.
Lyot, Kombut, Lend (B, Lyot,
Compt=Rend,)197.1953,193
3).

The pacing continuity of birefringence is due to the smooth birefringence profile.

This sharp peak in the birefringence profile results in significant output changes or discontinuities when the laser is tuned.

Figure 1 shows that the alexandrite birefringence spectrum at room temperature is smooth at wavelengths above 720 nm.

This desirable property is due to multiphonon processes within this spectral region at room temperature.
processes) are mainly included, and some multiphonons ((mu I t 1 phon
This is because the combination of (on) together produces birefringent fluorescence.

The peak, together with the density of photons in the state and the strength of the coupling, tend to eventually average out.

The tunable range of the laser of this invention is nominally 700
Between 820 nm and 820 nm, high excitation energies can be used to extend to longer wavelengths or conversely to shorter wavelengths.

For example, using high excitation energies, the R-line (6
A laser with a high wavelength (so, 4 nm) was obtained.

This wavelength range is also temperature dependent, with higher temperatures generally yielding longer wavelengths and lower temperatures shorter wavelengths (furthermore, this wavelength range can be determined using several nonlinear methods (although Frequency double and triple paced pumped parametric oscillators
ingand tripling, tuned-ex
citation pa -rametric o
scillators) and Raman frequency shifting (
Raman frequency shifting
) can also be extended.

All of these nonlinear frequency shifting methods provide a frequency-tuned output because the input frequency can be tuned.

Compared to ruby, the alexandrite birefringent laser of the present invention has a lower excitation threshold and a lower emission cross section (emission cross section).
Alexandrite has high mechanical strength and high thermal conductivity.

These properties allow for high average force operation, as well as high overall efficiency and high energy storage.

These also make the lasers of the invention attractive for high force applications such as welding, machining and fusing.

Another potentially useful property of the laser of the present invention is the existence of a two-part set of excitation levels with substantially different fluorescence properties.

This allows the laser's gain, bandwidth, and time to be thermally tailored to the specific application.

In particular, increasing the temperature of the laser medium also increases both gain and pacing range, and vice versa, at the expense of reduced storage time.

In operation, the laser of the present invention is used to create tunable coherent radiation.

The process of operating this laser consists of activating the light source to excite the laser medium and adjusting the tuning means, eg rotating the birefringent plate to obtain a predetermined output wavelength.

The laser optionally includes cooling means to control the temperature or maintain it at a predetermined temperature.

For example, if the cooling means includes a circulating fluid, the flow rate and temperature of this fluid may be adjusted to maintain a predetermined temperature.

The circulating fluid may be air, water, frozen fluid, etc., and preferably a heater is used to control the temperature of the fluid when required.

The present invention contemplates laser operation at temperatures ranging from about 77K to elevated temperatures of about 500C.

Surprisingly, tests tend to show that the gain of the laser increases at temperatures up to about 200°C.

Above 200° C., non-radiative etching will reduce the gain of the laser.

Thus, a preferred temperature range is between about ambient temperature and about 200°C.

The laser of the invention has potential applications in the communications field.

The laser of the present invention is tunable over a very wide wavelength range and can emit much more power than the diode lasers so often used for these purposes.

This laser crystal has a much longer useful life than a junction diode.

Because the output wavelength can be varied, it can be tuned away from the characteristic absorption in optical fibers used for communications, thus providing efficiency benefits.

In summary, the laser of the present invention offers a unique combination of advantages.

On the one hand, the present invention provides high average power, high efficiency and high energy storage, as well as Q-switching capability; on the other hand, the present invention provides wide wavelength tunability.

FIG. 2 illustrates the tunable laser device of the present invention.

Laser medium 1 made of alexandrite crystal of the present invention
1 and a pumping source 12 (e.g. continuous mercury or pulsed xenon gaseous emission pumping source)
It is contained within 0.

The container 10 has a highly reflective inner surface 13 with the contour of an elliptical depression.

The reflection at this surface 13 is diffuse or reflective.

The axes of the laser medium 11 and the pumping source 12 are each along the focal point of the ellipse formed by the container 10.

Laser medium 11 has coated ends 14 and 15 with conventional insulating antireflective coatings.

A fully reflective mirror 17 , a pacing element 20 and a partially reflective mirror 18 are placed outside the container 10 and approximately at the cylindrical axis of the laser medium 11 .

Laser action is confirmed by the emission of highly collimated coherent radiation, the wavelength of which is determined by the orientation of the tunable element 20.

Radiation indicated by arrow 16 emanates from a partially reflective mirror 18.

Both mirrors 17, 18 can be partially reflective.

If it is desired to obtain a predetermined operating temperature, the laser medium 11 and the pumping source 12 are cooled with a fluid circulating through the vessel 10.

This fluid may be air, water or a refrigerant and may optionally be preheated by conventional means to improve temperature control.

Optionally, the laser may include means for Q-switching.

These means can consist of a saturable dye absorber, an acousto-optical Q-switch or, as shown in FIG. 2, a polarizer 21 and a Pockels cell 22 placed in the path of the beam.

This polarizer 21 may be omitted, especially when the excitation power is low.

The Q-switch spoils (5poils) the Q of the cavity for the time that energy is being stored.

At an appropriate point in time, this Q-switch changes to a high gain state and the energy stored in the medium is suddenly released in di-iront pulses for a very short time.

This laser is also mode-locked (mode 1oo
ked).

As shown in FIG. 3, an amplifier can be used with the apparatus of FIG. 2 in the high power laser system of the present invention.

The device of FIG. 2 can be considered the oscillator of this amplifier.

This amplifier is placed at the exit beam of the oscillator.

It consists essentially of a container 30 having a fully reflective inner surface 33 contoured with an elliptical depression.

Amplifier rod 3 excited by flash lamp 32
1 is a coated end 3 with a conventional insulating anti-reflective coating.
4.35.

This amplifier rod has a larger diameter than the oscillator rod 11, in this case a beam expanding telescope.
e) 36 is placed between the amplifier and the oscillator to match the beam size to the rod size.

Unlike an oscillator, this amplifier typically does not have mirrors at its ends to form a dimple, and the amplification of the oscillator's output occurs during one pass of the laser beam through the amplifier rod.

However, in some applications, the amplifier is equipped with a partially reflecting mirror to provide a back part of the amplifier's output to the amplification medium.

The spectroscopic and temporal characteristics of the output of this regenerative oscillator can then be determined by interpolating a suitably matched signal from the original amplifier in the same way as with a single-pass amplifier.

This amplifier can use several of these.

Amplified power, indicated by arrow 37, radiates from amplifier rod 31.

FIG. 4 is a plot of the relationship between threshold energy and wavelength of a pulsed laser.

This diagram illustrates the wide range of continuous tunability and relatively low threshold energy of the tunable laser of the present invention.

FIG. 5 shows the dependence of the output power on wavelength for a constant continuous pump power of the tunable laser of the present invention.

The wavelength range is shown to be somewhat narrower than for the pulsed mode.

However, this simply reflects the generally lower efficiency of continuous operation.

FIG. 6 shows that the emission of the 750 nm and 780 nm cross-sections, and thus the laser gain of the laser of the present invention, increases with temperature.

This behavior is rare in lasers and allows for improved energy extraction in Q-switch and amplifier operation.

Embodiment Embodiment 1 shows a conventional alexandrite laser that does not use a tuning means.

Other embodiments are illustrative of the tunable laser of the present invention.

1. A single crystal of (001) (cm-axis) oriented alexandrite containing 0.02-0.03 atomic percent Cr was placed in an iridium crucible under a flowing nitrogen atmosphere using the Czochralski method. Made from stoichiometric amounts of melt.

C- with dimensions of length 7.5 cm and diameter 0.63 cfrL
A shaft roll was made from the above crystal.

The rod was pumped with a single linear xenon flash lamp located in a water-cooled, silver-coated, oval cross-section cavity (which had an active length of 5.72 m).

The duration of the flash was 100 μs.

720Hm and 726nm at 35J input
Laser emission was observed during this period.

When pumped to 50 J, the laser emission was slightly shifted to shorter wavelengths.

The 28J threshold was observed using a mirror that reflects 98% at the laser frequency.

2. C-axis single crystal Rolo of alexandrite containing 0.03-0.04 atomic percent cr3+ has 30
The output reaches 0 mJ.

The efficiency of the slope is about 0.38%, and the effective emission cross section entailed by the laser test is 1.04X10 cr at the optimal laser frequency of 750 nm.
It was it.

The laser consists of three parallel birefringent plates mounted within an optical cavity at a B rewster's angle to the light reflected within this cavity.
701nm and 794nm using otJ birefringence filter
Continuously tuned between nm.

The laser oscillator threshold as a function of wavelength in this arrangement is shown in FIG.

3. Single crystal rod of alexandrite (0.3 cII diameter) containing about 0.2 atomic percent Cr3+
LX length 7 CrrL) was placed on the axis along the focal line of one of the water-cooled, silver-coated, oval cross-section pump recesses.

A 3 kW ac mercury arc lamp was placed on axis along the conjugate focal axis of this cavity.

convert the lamp into a ballast transformer
(ransformer).

98 type reflective mirror with 4m concave front surface 30.5c
The rfL separation formed a stable resonator from which lasing was obtained at the peaks of the pump light curve.

This laser uses a Lyok birefringence filter to
It was continuously tuned between 783 nm and 783 nm.

0.0 at 756 nm. A maximum output with a wavelength of 02 nm occurred.

The threshold for laser action at 756 nm is approximately 1.5k
W, and the slope efficiency was approximately o,s%.

The maximum output power was 6.5w. 4. C-axis single crystal lolo of alexandrite containing 0.12 atomic percent Cr3+ (diameter 0.64CfrLX length 9
．． 6 am) was pumped with a water-cooled, silver-coated, linear xenon flash lamp in an oval cross-section cavity (length 76 cIIL).

The end mirrors were separated by about 30α. The ends of this rod are uncoated and have a reflectance of about 7.

Mirrors (one flat and one with a 4m concave surface)
were arranged parallel to the plane of the rod.

The temperature dependence of both the output and the threshold value was determined in the range of 17°C to 65°C.

Within this range, the increase in power (constant input) and decrease in threshold were substantially linear with increasing temperature.

[Brief explanation of the drawing]

FIG. 1 shows the room temperature fluorescence spectrum of alexandrite used in the present invention. FIG. 2 is a partial cross-sectional view of a typical laser device employing a laser rod and tunable means to provide the tunable laser of the present invention. FIG. 3 is a schematic diagram of an oscillator-amplifier laser system. FIG. 4 is a plot of the wavelength of the tunable laser of the present invention versus the threshold energy of the pulsed laser. FIG. 5 is a plot of constant continuous input power wavelength versus output power for the tunable laser of the present invention. FIG. 6 is a plot of the temperature dependence of the effective emission cross section of a tunable laser of the invention emitting laser radiation at 750 and 780 nm.

Claims (1)

[Claims] 1 a, about 0.01 for aluminum sites
a laser medium consisting of a single crystal of beryllium aluminate of chrysoberyl structure having a trivalent chromium doping concentration of ~10 atomic percent; b. excitation of said laser medium to emit coherent radiation; and C. tunable means for pacing said coherent radiation in the spectral range from the deep red region to the infrared region. 2. The laser of claim 1, wherein the chromium doping concentration is in the range of about 0.01 to 1.0 atomic percent. 3. A laser according to claim 1, wherein the chromium doping concentration is in the range of 0.1 to 0.4 atomic percent. 4. Claim 1, wherein the coherent radiation propagates in a direction substantially along the a-c plane of the crystal with a polarization electric vector substantially perpendicular to said a-c plane.
Laser as described in section. 5. A laser according to claim 1, wherein the tuning means is selected from the group consisting of birefringent filters, prisms, optical gratings, multilayer insulation coated filters, and lenses with axial chromatic aberration. 6. The laser of claim 1, wherein the means for exciting the laser medium comprises a pulsed or continuous coherent or incoherent light source emitting at a wavelength shorter than about 700 nm. 7. The light source is a xenon or mercury arc lamp. A laser according to claim 6 consisting of: 8. The laser of claim 1 further comprising means for Q-switching the laser. 9. The laser of claim 1 further comprising means for cooling the laser medium.