CA2026146A1 - Single-frequency laser of improved amplitude stability - Google Patents
Single-frequency laser of improved amplitude stabilityInfo
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- CA2026146A1 CA2026146A1 CA 2026146 CA2026146A CA2026146A1 CA 2026146 A1 CA2026146 A1 CA 2026146A1 CA 2026146 CA2026146 CA 2026146 CA 2026146 A CA2026146 A CA 2026146A CA 2026146 A1 CA2026146 A1 CA 2026146A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/102—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
- H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
- H01S3/109—Frequency multiplication, e.g. harmonic generation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
- H01S3/131—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/102—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
- H01S3/1028—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation by controlling the temperature
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Plasma & Fusion (AREA)
- Optics & Photonics (AREA)
- Nonlinear Science (AREA)
- Lasers (AREA)
Abstract
Abstract of the Disclosure A solid-state laser of improved amplitude stability is obtained by substantially eliminating spatial hole burning in the lasant material and maintaining the optical cavity of the laser at a temperature which results in sub-stantially noise-free generation of output radiation.
Description
SINGLE-FREQUENCY LASER OF IMPROVED
AMPLITUDE STABILITY
Field of the Invention This invention relates to a method for optimizing the amplitude stability of a solid-state laser.
Cross-Reference to Related Applications This application is a continuation in-part of apply-cation Serial No. 353,870, filed May 18, 1989, which is acontinuation-in-part of application Serial No. 207, 666, filed June 16, 1988, which in turn is a continuation-in-part of application Serial No. 157,741, filed February 18, 1988.
Background of the Invention A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated ems-20 soon of photons from atoms, molecules or ions of an active medium which have typically been excited from a ground state to a higher energy level by an input of energy.
Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transit lion to a lower energy state can stimulate the emission often of substantially identical energy from other excited atoms, molecules or tong. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. A
portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission I
through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical-pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as laser diodes, are typically constructed from materials such as gallium arsenide and aluminum gal-ilium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is rota-lively high and, for example, can be in excess of 40 per-cent.
The use of flash lamps, light-emitting diodes (as used herein, this term includes super luminescent diodes and super luminescent diode arrays) and laser diodes (as used herein, this term includes laser diode arrays) to optically pump or excite a solid lasant material is well-known. Lasant materials commonly used in such solid state lasers include crystalline or glassy host materials into which an active material, such as trivalent neodymium ions, is incorporated. Highly suitable solid lasant mate-fiats include substances wherein the active material is a stoichiometric component of the lasant material. Such stoichiometric materials include, for example, neodymium pentaphosphate and lithium neodymium tetraphosphate.
Detailed summaries of conventional solid lasant materials are set forth in the CRC Handbook of Laser Science and Technology, Vol. I, M. J. Weber, Ed., CRC Press, Inc., coca Rayon, Florida, 1982, pp. 72-135 and by A. A. Gamin-skit in Laser Crystals, Vol. 14 of the Springer Series in Optical Sciences, D. L. Macadam Ed., Springer-Verlag, New York, NAY., 1981. Conventional host materials for needy-I
mum ions include glass, yttrium aluminum guaranty, referred to as YAW), YO-YO (referred to as YALE), Luff (referred to as ELF), and gadolinium scandium gallium garnet (Gd3Sc2Ga3O12, referred to as GSGG). By way of example, when neodymium-doped YAW is employed as the lasant material in an optically-pumped solid state laser, it can be pumped by absorption of light having a wavelength of about 808 no and can emit light having a wavelength of 1064 no.
U.S. Patent No. 3,624,545 issued to Ross on Novel-bier 30, 1971, describes an optically-pumped solid state laser composed of a YAW rod which is side-pumped by at least one semiconductor laser diode. Similarly, U.S.
Patent No. 3,753,145 issued to Chester on August 14, 1973, discloses the use of one or more light-emitting semi con-doctor diodes to end-pump a neodymium-doped YAW rod. The use of an array of pulsed laser diodes to end-pump a solid lasant material such as neodymium-doped YAW is described in U.S. Patent No. 3,982,201 issued to Rosenkrantz et at.
on September 21, 1976. Finally, D. L. Swipes, Apply Pays.
Let., Vol. 47, No. 2, 1985, pp. 74-75, has reported that the use of a tightly focused semiconductor laser diode array to end pump a neodymium-doped YAW results in a high efficiency conversion of pumping radiation having a wave-length of 810 no to output radiation having a wavelength of 1064 no.
Solid-state lasers which exhibit single longitudinal mode operation to yield a single-frequency output can be obtained by eliminating spatial hole burning in the lasant material. Spatial hole burning is a consequence of the electric field nodes that are associated with a linearly polarized standing wave. The population inversion in the lasant material at these nodes does not contribute to the standing wave and will preferentially contribute to other longitudinal modes. In gas lasers, spatial hole burning is substantially prevented by the thermal motion of the atoms and/or molecules of the gas. However, in solids the motion of atoms and/or molecules is small in comparison with the wavelength of the light produced by the laser, and spatial averaging cannot take place.
Spatial hole burning in a solid-state laser can be prevented by eliminating the accumulation of an unutilized population inversion in the lasant material at the nodes of a standing wave. As a consequence, alternative long-tudinal modes are prevented from reaching threshold, and the laser can produce single-frequency output through single longitudinal mode operation. For example, spatial hole burning can be eliminated through the use of a unwooed-sectional ring cavity, by generating circularly polarized light in the lasant material, with mechanical motion, or with electro-optic phase modulations.
The use of a unidirectional ring cavity to prevent spatial hole burning is summarized in W. Cautionary, Solid-State Laser Engineer no (Springer-Verlag, New York, Second Ed., 1988) at pp., 126-128 and 223, and in Siegman, Lasers university Science Books, Mill Valley, California a, 1986) at pp. 532-538. U.S. Patent No. 4,749,842 issued to Kane et at. on June 7, 1988 describes a laser diode-pumped, monolithic, unidirectional, ring laser wherein the lasant material is in thermal contact with a heating element.
This patent discloses that the output wavelength of a monk olithic, solid-state, unidirectional, ring laser can be temperature tuned by means of a heating element.
The elimination of spatial hole burning by generating circularly polarized light in the lasant material is described by V. Evtuhov et at., Apply-. Optics, Vol. 4, No. 1, pp. 142-143 (1965). In this approach, an axially uniform energy density is created in the lasant material by forcing the laser mode to be a circularly polarized standing wave. The standing-wave electric field vector of such a mode changes in direction but not in magnitude as a function of position in the lasant material. Accordingly, there are no electric field nodes to cause spatial hole I
burning. A laser of this type is commonly referred to as a "twisted-mode" device.
A twisted-mode, single-frequency, neodymium-doped YAW
laser has been reported by D. A. Draegert, IEEE J. Quantum Electronics, Vol. QUEUE, No. 2, February 1972, pp. 235-239.
The laser described in this report was optically-pumped with a tungsten-iodine lamp and contained within its optical cavity a neodymium-doped YAW rod positioned between two quarter-wave plates together with a Brewster's angle plate placed immediately in front of one of the two end mirrors of the cavity. The fast axes of the two quart ter-wave plates were perpendicular to each other and oft-enter at a 45 angle to the direction of polarization determined by the Brewster's angle plate. In addition, it is stated that this twisted-mode technique should work well with small diode-pumped neodymium-doped YAW lasers.
More recently, a twisted-mode, single-frequency, laser diode-pumped, neodymium-doped YAW laser has been described at pp. 38-40 of Laser Focus World (April 1989).
It is known in the art that amplitude and frequency instabilities can be observed in the output of single-fre-quench solid-state lasers. For example, such instable-ties are discussed by Daniel Meyer, IEEE J. Quantum Electronics, Vol. QUEUE, No. 2, February 1970, pp. 101-104.
Summary of the Invention The present invention is directed to a method for improving the amplitude stability of a solid-state laser.
A high degree of amplitude stability is desirable if the output radiation from such a laser is to be used in apply-cations such as optical storage of data, spectroscopy, communications, sensor systems, projection displays, laser printing and laser film read/write systems.
A solid-state laser which typically emits output radiation of a single-frequency can be obtained by elm-noting spatial hole burning in the lasant material. We have found that the amplitude fluctuations (noise) in the I I
output radiation from such a device are a function of them-portray and drop to very low values over certain well-de-fined temperature ranges or "temperature windows."
Accordingly, we have discovered that substantially noise-S free operation can be achieved by maintaining the temper-azure of the laser cavity at a value within such a temperature window.
One embodiment of the invention is a method for gent crating coherent optical radiation which comprises:
(a) generating coherent optical radiation from solid lasant material within an optical cavity for said radix anion; (b) substantially preventing spatial hole burning in the lasant material during the generation of said radix anion with spatial hole burning suppression means;
(c) withdrawing at least a portion of said radiation from the optical cavity as output; and (d) maintaining the them-portray of the optical cavity within a range over which said output is substantially noise-free.
Another embodiment of the invention is an optically-pumped solid-state laser comprising: (a) a standing wave optical cavity; (b) optical pumping means for generating optical pumping radiation, wherein said optical pumping means comprises a laser diode; (c) solid lasant material which is disposed within said optical cavity, positioned to receive pumping radiation from said optical pumping means, and effective for generating coherent optical radix anion upon receiving the pumping radiation; (d) means log substantially eliminating spatial hole burning in the lasant material during said generation of coherent optical radiation; and (e) temperature control means for maintain-in the temperature of the optical cavity at a value which results in substantially noise free generation of said coherent radiation.
An object of the invention is to provide a laser diode-pumped solid state laser of improved amplitude stay ability.
2~2~
Another object of the invention is to provide a laser diode-pumped, twisted-mode, solid-state laser of improved amplitude stability.
A further object of the invention is to provide a method for reducing the amplitude instabilities that are observed in the output of a solid-state laser.
A still further object of the invention is to provide a method for improving the amplitude stability of a laser diode-pumped, twisted-mode, solid-state laser.
Brief Description of the Draw nags FIG. 1 of the drawings is a schematic illustration of one embodiment of the invention.
FIG. 2 of the drawings illustrates the optical output power of a laser diode-pumped solid-state laser of the type illustrated by FIG. 1 and the root mean square (RUMS) noise content of said output as a function of the temper-azure of the laser cavity.
FIG. 3 of the drawings is a schematic illustration of a second embodiment of the invention.
Detailed Description of the Preferred Embodiments While this invention is susceptible of embodiment in many forms, two specific embodiments are schematically shown in FIGS. 1 and 3 of the drawings, with the under-standing that the present disclosure is not intended to limit the invention to the embodiments illustrated.
With reference to FIG. 1, optical pumping radiation 1 from optical pumping means 2 and 3 is focused by focusing means 4 through quarter-wave plate 5 and into solid lasant material 6 which is capable of being pumped by the radix anion from said pumping means (2 and 3). Light emitted by the losing of lasant material 6 oscillates within the linear standing wave optical cavity defined by mirror 7 and a suitable reflective coating on surface 8 of quart 2~2~
I
ter-wave plate 5, and such light is hereinafter referred to as cavity radiation. A second quarter-wave plate 9 and a Brewster plate polarizer 10 are positioned between lasant material 6 and mirror 7. The pair of quarter-wave plates 5 and 9 serves to suppress spatial hole burning within lasant material 6, and the Brewster plate polarizer 10 causes the cavity radiation to be linearly polarized at mirror 7 and also determines the direction of polarize-lion. A portion of the cavity radiation is passed through mirror 7 as output radiation 11. Finally, the laser illustrated in FIG. 1 is provided with temperature control means which is not shown in the figure) for adjusting and controlling the temperature of the laser's optical cavity and its contents.
Optical pumping means 2 and 3 can comprise any con-ventional source of optical pumping radiation. However, preferred sources of pumping radiation 1 include light-em-witting diodes and laser diodes. Such diodes are commonly attached to a thermally conductive heat sink and are pack-aged in a metal housing. For efficient operation, the pumping radiation 1 is matched with a suitable absorption band of the lasant material 6. Conventional light-emit-tying diodes and laser diodes are available which, as a function of composition, produce output radiation having a wavelength over the range from about 630 no to about 1600 no, and any such device producing pumping radiation 1 of a wavelength effective to pump lasant material 6 can be used in the practice of this invention. For example, the wavelength of the output radiation from a Gain based device can be varied from about 630 to about 700 no by variation of the device composition. Similarly, the wave-length of the output radiation from a Galas based device can be varied from about 750 to about 900 no by variation of the device composition, and InGaAsP based devices can be used to provide radiation in the wavelength range from about 1000 to about 1600 no.
A highly suitable source of optical pumping radiation 1 consists of a gallium aluminum arsenide laser diode array 3, emitting light having a wavelength of about 810 no, which is attached to heat sink 2. Heat sink 2 can be passive in character. However, heat sink 2 can also comprise a thermoelectric cooler to help maintain laser diode array 3 at a constant temperature and thereby ensure optimal operation of laser diode array 3 at a constant wavelength. It will be appreciated, of course, that during operation the optical pumping means will be attached to a suitable power supply. Electrical leads from laser diode array 3 which are directed to a power supply are not illustrated in FIG. 1.
Focusing means 4 serves to focus pumping radiation 1 through quarter-wave plate 5 and into lasant material 6.
This focusing results in a high pumping intensity and an associated high photon to photon conversion efficiency in lasant material 6. hocusing means 4 can comprise any con-ventional means for focusing light such as a gradient index lens, a ball lens, an spheric lens or a combination of lenses.
Any conventional solid lasant material 6 can be Utah-lived provided that it is capable of being optically pumped by the optical pumping means selected. Suitable lasant materials include, but are not limited to, solids selected from the group consisting of glassy and crystal-line host materials which are doped with an active mate-fiat and substances wherein the active material is a stoichiometric component of the lasant material. Highly suitable active materials include, but are not limited to, ions of chromium, titanium and the rare earth metals.
Highly suitable lasant materials include neodymium-doped YAW, neodymium-doped YALE, neodymium-doped ELF, neodymi-um-doped GSGG, neodymium pentaphosphate and lithium needy-mum tetraphosphate. my way of specific example,neodymium-doped YAW is a highly suitable lasant material 6 for use in combination with an optical pumping means which produces light having a wavelength of about 810 no. When pumped with light of this wavelength, neodymium-doped YAW
can emit light having a wavelength of 1064 no.
The precise geometric shape of lasant material 6 can vary widely. For example, lasant material 6 can be rod-shaped, or rhombohedral in shape if desired, and lens-shaped surfaces can be used if desired. If desired, an end-pumped fiber of lasant material can be used. Highly suitable fibers for this purpose include, but are not limp tied to, glass optical fibers which are doped with ions ova rare earth metal such as neodymium. The length of such a fiber is easily adjusted to result in absorption of essentially all of the optical pumping radiation 1. If a very long fiber is required, it can be coiled, on a spool for example, in order to minimize the overall length of the laser of this invention.
The reflective coating on surface 8 of quarter-wave plate 5 is selected in such a manner that it is sub Stan-tidally transparent to optical pumping radiation 1 but highly reflective with respect to the cavity radiation produced by the losing of lasant material 6.
Mirror 7 is selected in such a manner that it is par-tidally transmitting for the cavity radiation produced by the losing of lasant material 6. Mirror 7 is conventional in character and, for example, can comprise any suitable conventional coating on any suitable substrate.
The pair of quarter-wave plates 5 and 9 are quarter-wave plates for cavity radiation and serve as a means for substantially eliminating spatial hole burning by causing circular polarization of the cavity radiation within lasant material 6 [this technique for producing an axially uniform energy density in a lasant material is described by V. Evtuhov et at., Apply Optics, Vowel, Noel, pp. 142-143 (1965)]. The precise location of these two quarter-wave plates within the optical cavity is not critical provided that lasant material 6 is placed between them.
Brewster plate polarizer 10 results in the linear polarization of cavity radiation at mirror 7 and also determines the direction of polarization. This polarize-lion permits convenient optimization of the orientation of quarter-wave plates 5 and 9 for elimination of spatial hole burning in lasant material 6. However, Brewster plate polarizer 10 is not an essential element of the invention and is not required to effect substantial elm-nation of spatial hole burning with quarter-wave plates 5 and 9.
A highly satisfactory orientation for quarter-wave plates 5 and 9 and for Brewster plate polarizer 10 involves positioning the quarter-wave plates so that the optic axis of one is substantially perpendicular with respect to the optic axis of the other about the axis along which they encounter cavity radiation and position-in Brewster plate polarizer 10 so that its highly trays-milting axis is oriented at an angle of about 45 with respect to the optic axis of each quarter-wave plate.
Any conventional means for substantially eliminating spatial hole burning in the lasant material can be used in the practice of this invention. For example, spatial hole burning can be eliminated through the use of a traveling-wave ring-like optical cavity (which is illustrated by the embodiment set forth in FUGUE), by generating circularly polarized light in the lasant material (which is thus-treated in FIG. 1 and is effected by the pair of quarter-wave plates 5 and 9), with mechanical motion, or with electro-optic phase modulation.
The laser illustrated by FIG. 1 is provided with them-portray control means for adjusting and controlling the temperature of the laser's optical cavity and its con-tents. This temperature control means can be of any con-ventional type, for example, an electrically powered resistance heater or thermoelectric device, and is used to maintain the temperature of the optical cavity at a value which results in substantially noise-free generation of 2~2~
output radiation 11. Through the use of such temperature control means, the temperature of the cavity and its con-tents is desirably maintained at `1C of the selected value, preferably at `0.5C of the selected value and more preferably at `0.1C of the selected value.
The undesired fluctuations in the amplitude of output radiation 11, for example, within a frequency range of about 1 kHz to about 50 MHz, are referred to as noise and are conveniently measured as percent root mean square (%
RUMS) noise. Although a laser of improved amplitude stay ability is obtained through the substantial elimination of spatial hole burning in the lasant material 6, we have found that the noise content of output radiation 11 drops essentially to zero over certain temperature ranges. More specifically, if the noise content of output radiation 11 is measured as a function of the temperature of the laser cavity, certain ranges of temperature or "temperature win-dowse are observed over which the noise drops to Essex-tidally zero. The width of these windows and the precise temperatures at which they occur are different for each individual laser. That is to say, we have found that the noise content of output radiation if as a function of the temperature of the laser cavity is a unique characteristic of each device. However, for a given laser, the noise content of the output radiation 11 as a function of them-portray does not change significantly over long periods of time (for example, weeks or months) or with repeated cycling over large temperature ranges. Accordingly, a highly preferred embodiment of this invention comprises locating a window of substantially noise-free operation for a laser by measuring the noise content of output radix anion 11 as a function of the temperature of the optical cavity and maintaining the temperature of the laser cavity at a value within such a window during subsequent opera-lion. These windows of substantially noise-free operation are typically from about 1 to about 15C wide, and within such a window the RUMS noise will typically be less than about 0.2% and frequently less than about 0.1%. The win-dowse of substantially noise-free operation are easily identified by measuring the noise content of output radix anion 11 over any range of temperature which is convenient from an operating point of view, for example, from about 0 to about 100C, or more conveniently, from about 30 to about 65C. This measurement is desirably carried out over a range of at least about 5C and preferably over a range of at least about 10C or 20C in order to give a reasonable sampling of the laser's temperature-related performance.
In a specific example of the embodiment illustrated in FIG. 1, neodymium-doped YAW is used as lasant material 6. The neodymium-doped YAW is optically pumped by a mull tistripe laser diode array 3 which is attached to a then-moelectric cooler 2 (the array and attached thermoelectric cooler is a Model SOL 2422-Hl device manufactured by Specs ire Diode Labs of San Jose, California). The laser diode array 3 is a 10-stripe array consisting of 3 micron stripes on 10 micron centers which can provide about 200 my of pumping radiation 1 having a wavelength of about 810 no. This pumping radiation 1 is focused by gradient index lens 4 which has a 0.29 pitch and is anti reflection coated with respect to 810 no wavelength radiation. The focused pumping radiation passes through quarter-wave retardation plate 5 which is comprised of quartz and is in the form of a circular plate having a thickness of about 1 mm and a 10 mm diameter. Input face 8 of quarter-wave plate 5 carries a multi layer dielectric coating which is highly reflective (R > 99.8~) at a wavelength of 1064 no and highly trays-parent (T > 80%) at a wavelength of 810 no. Output face 12 of ~uarter-wave plate 5 carries an anti reflection coat-in (R < 0.2~) for light having a wavelength of 1064 no and highly transparent (T > 80%) at a wavelength of 810 no. The focused pumping radiation comes to a focus within lasant material 6 which contains about I neodymium and is in the form of a disc having a 4 mm thickness and a 10 mm diameter. The lasant material 6 is oriented for low threshold operation at a wavelength of 1064 no and emits light (cavity radiation) having a wavelength of 1064 no in response to excitation by the pumping radiation. The sun-faces of lasant material 6 are anti reflection coated (R <
0.2%) for light having a wavelength of 1064 no and highly transparent (T > 80%) at a wavelength of 810 no. Quart ter-wave plate 9 is identical with plate 5 except that both of its surfaces are anti reflection coated with respect to 1064 no wavelength radiation. Quarter-wave plates 5 and 9 are positioned in such a manner that the optic axis of one makes a 90 angle with respect to the optic axis of the other about the axis along which they encounter cavity radiation. The highly transmitting axis lo of Brewster plate polarizer 10 is oriented so that it makes an angle of 45 with respect to the optic axis of each of the quarter-wave plates 5 and 9 about the axis along which these components encounter cavity radiation.
Output radiation 11 having a frequency of 1064 no is tray-spitted through mirror 7 which has a radius of curvature of 30 cm and carries a dielectric coating which is 97%
reflective at a wavelength of 1064 no. The optical cavity of this laser has a length (distance from surface 8 to mirror 7) of about 20 mm. The optical cavity is wrapped I with an electrically powered resistance heater and fitted with a thermistor which can be used to: (a) measure the cavity temperature; and (b) control the cavity temperature by providing a feedback signal to the power supply for the resistance heater.
The power and percent root mean square (RUMS) noise of the 1064 no output radiation from the above-described laser were measured a a function of temperature over the range from about 40 to about 60C. The results are set forth in FIG. 2. With reference to FIX. I, it will be noted that the RUMS noise in the output radiation drops to very low values over certain ranges of temperature or "temperature windows." For example, such a window for single frequency operation appears at about 40.0~43.0C.
The precise location and width of these windows of sub-staunchly noise-free operation are unique characteristics of each individual laser and do not change significantly with time (for example, weeks or months) or repeated them-portray cycling over several tens of degrees centigrade.
Accordingly, each laser can be made to operate in a sub-staunchly noise-free manner by maintaining the optical cavity of the device at a temperature within such a window during operation.
It will also be noted from FIG. 2 that the laser functions in a multi longitudinal mode manner (multimedia operation) over certain temperature ranges. This is typic eel behavior for a solid-state laser in which spatial hole burning has been suppressed, and a preferred embodiment of the invention involves selecting a temperature window which yields: (a) substantially noise-free operation, and (b) single transverse mode operation.
FIG. 3 schematically illustrates a second embodiment of the invention which involves the use of a traveling-wave ring-like optical cavity for the purpose of sub Stan-tidally eliminating spatial hole burning in the lasant material. With reference to FIG. 3, optical pumping radix anion 21 from laser diode array 22 is focused by focusing means 23 into lasant material 24 which is capable of being pumped by said pumping radiation. Laser diode array 22 is attached to heat sink 25. Light emitted by the losing of lasant material 24 is contained within the optical cavity defined by mirrors 26, 27 and 28 and by a suitable reflect live coating on surface 29 of lasant material 24, and sociality is hereinafter referred to as cavity radiation. A
unidirectional optical gate means for effecting unidirec-tonal circulation of cavity radiation within the optical cavity is provided by the combination of polarizer 30, Faraday rotator 31, and half-wave plate 32. A portion of the cavity radiation is passed through mirror I as output radiation 33. Finally, the laser illustrated in FIG. 3 is provided with temperature control means (which is not shown in FIG. 3) for adjusting and controlling the temper-azure of the optical cavity and its contents. This them-portray control means is employed to maintain the optical cavity of the laser at a value which results in sub Stan tidally noise-free generation of output radiation 33.
The reflective coating on surface 29 of lasant mate-fiat 24 is selected in such a manner that it is sub Stan-tidally transparent to optical pumping radiation 21 but highly reflective with respect to the cavity radiation produced by the losing of lasant material 24. Mirrors 26 and 27 are highly reflective for the cavity radiation pro-duped by the losing of lasant material I Mirror 28 is partially transmitting for cavity radiation.
Any conventional polarization means can be utilized as polarizer 30, for example, a Brewster plate, suitable coatings on the mirrors of the optical cavity, a Delco-trig polarizer, or a Brewster angle surface on the lasant material 24.
If neodymium-doped YAW is used as the lasant material 24, the YAW crystal itself can also serve as Faraday rota-ion 31 if a magnetic field is established along the axis of the crystal. In such an embodiment, a separate Faraday rotator 31 is not required.
Conventional designs for a traveling-wave optical cavity which can be employed in the practice of this invention for the purpose of substantially eliminating spatial hole burning in the last material are set forth in W. Cautionary, Solid-State Laser Engineering (Springer-30 Verlag, New York, Second Ed., 1988) at pp. 126-128 and in Siegman, Lasers (University Science Books, Mill Valley, California, 1986) at pp. 532-538.
AMPLITUDE STABILITY
Field of the Invention This invention relates to a method for optimizing the amplitude stability of a solid-state laser.
Cross-Reference to Related Applications This application is a continuation in-part of apply-cation Serial No. 353,870, filed May 18, 1989, which is acontinuation-in-part of application Serial No. 207, 666, filed June 16, 1988, which in turn is a continuation-in-part of application Serial No. 157,741, filed February 18, 1988.
Background of the Invention A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated ems-20 soon of photons from atoms, molecules or ions of an active medium which have typically been excited from a ground state to a higher energy level by an input of energy.
Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transit lion to a lower energy state can stimulate the emission often of substantially identical energy from other excited atoms, molecules or tong. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. A
portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission I
through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical-pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as laser diodes, are typically constructed from materials such as gallium arsenide and aluminum gal-ilium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is rota-lively high and, for example, can be in excess of 40 per-cent.
The use of flash lamps, light-emitting diodes (as used herein, this term includes super luminescent diodes and super luminescent diode arrays) and laser diodes (as used herein, this term includes laser diode arrays) to optically pump or excite a solid lasant material is well-known. Lasant materials commonly used in such solid state lasers include crystalline or glassy host materials into which an active material, such as trivalent neodymium ions, is incorporated. Highly suitable solid lasant mate-fiats include substances wherein the active material is a stoichiometric component of the lasant material. Such stoichiometric materials include, for example, neodymium pentaphosphate and lithium neodymium tetraphosphate.
Detailed summaries of conventional solid lasant materials are set forth in the CRC Handbook of Laser Science and Technology, Vol. I, M. J. Weber, Ed., CRC Press, Inc., coca Rayon, Florida, 1982, pp. 72-135 and by A. A. Gamin-skit in Laser Crystals, Vol. 14 of the Springer Series in Optical Sciences, D. L. Macadam Ed., Springer-Verlag, New York, NAY., 1981. Conventional host materials for needy-I
mum ions include glass, yttrium aluminum guaranty, referred to as YAW), YO-YO (referred to as YALE), Luff (referred to as ELF), and gadolinium scandium gallium garnet (Gd3Sc2Ga3O12, referred to as GSGG). By way of example, when neodymium-doped YAW is employed as the lasant material in an optically-pumped solid state laser, it can be pumped by absorption of light having a wavelength of about 808 no and can emit light having a wavelength of 1064 no.
U.S. Patent No. 3,624,545 issued to Ross on Novel-bier 30, 1971, describes an optically-pumped solid state laser composed of a YAW rod which is side-pumped by at least one semiconductor laser diode. Similarly, U.S.
Patent No. 3,753,145 issued to Chester on August 14, 1973, discloses the use of one or more light-emitting semi con-doctor diodes to end-pump a neodymium-doped YAW rod. The use of an array of pulsed laser diodes to end-pump a solid lasant material such as neodymium-doped YAW is described in U.S. Patent No. 3,982,201 issued to Rosenkrantz et at.
on September 21, 1976. Finally, D. L. Swipes, Apply Pays.
Let., Vol. 47, No. 2, 1985, pp. 74-75, has reported that the use of a tightly focused semiconductor laser diode array to end pump a neodymium-doped YAW results in a high efficiency conversion of pumping radiation having a wave-length of 810 no to output radiation having a wavelength of 1064 no.
Solid-state lasers which exhibit single longitudinal mode operation to yield a single-frequency output can be obtained by eliminating spatial hole burning in the lasant material. Spatial hole burning is a consequence of the electric field nodes that are associated with a linearly polarized standing wave. The population inversion in the lasant material at these nodes does not contribute to the standing wave and will preferentially contribute to other longitudinal modes. In gas lasers, spatial hole burning is substantially prevented by the thermal motion of the atoms and/or molecules of the gas. However, in solids the motion of atoms and/or molecules is small in comparison with the wavelength of the light produced by the laser, and spatial averaging cannot take place.
Spatial hole burning in a solid-state laser can be prevented by eliminating the accumulation of an unutilized population inversion in the lasant material at the nodes of a standing wave. As a consequence, alternative long-tudinal modes are prevented from reaching threshold, and the laser can produce single-frequency output through single longitudinal mode operation. For example, spatial hole burning can be eliminated through the use of a unwooed-sectional ring cavity, by generating circularly polarized light in the lasant material, with mechanical motion, or with electro-optic phase modulations.
The use of a unidirectional ring cavity to prevent spatial hole burning is summarized in W. Cautionary, Solid-State Laser Engineer no (Springer-Verlag, New York, Second Ed., 1988) at pp., 126-128 and 223, and in Siegman, Lasers university Science Books, Mill Valley, California a, 1986) at pp. 532-538. U.S. Patent No. 4,749,842 issued to Kane et at. on June 7, 1988 describes a laser diode-pumped, monolithic, unidirectional, ring laser wherein the lasant material is in thermal contact with a heating element.
This patent discloses that the output wavelength of a monk olithic, solid-state, unidirectional, ring laser can be temperature tuned by means of a heating element.
The elimination of spatial hole burning by generating circularly polarized light in the lasant material is described by V. Evtuhov et at., Apply-. Optics, Vol. 4, No. 1, pp. 142-143 (1965). In this approach, an axially uniform energy density is created in the lasant material by forcing the laser mode to be a circularly polarized standing wave. The standing-wave electric field vector of such a mode changes in direction but not in magnitude as a function of position in the lasant material. Accordingly, there are no electric field nodes to cause spatial hole I
burning. A laser of this type is commonly referred to as a "twisted-mode" device.
A twisted-mode, single-frequency, neodymium-doped YAW
laser has been reported by D. A. Draegert, IEEE J. Quantum Electronics, Vol. QUEUE, No. 2, February 1972, pp. 235-239.
The laser described in this report was optically-pumped with a tungsten-iodine lamp and contained within its optical cavity a neodymium-doped YAW rod positioned between two quarter-wave plates together with a Brewster's angle plate placed immediately in front of one of the two end mirrors of the cavity. The fast axes of the two quart ter-wave plates were perpendicular to each other and oft-enter at a 45 angle to the direction of polarization determined by the Brewster's angle plate. In addition, it is stated that this twisted-mode technique should work well with small diode-pumped neodymium-doped YAW lasers.
More recently, a twisted-mode, single-frequency, laser diode-pumped, neodymium-doped YAW laser has been described at pp. 38-40 of Laser Focus World (April 1989).
It is known in the art that amplitude and frequency instabilities can be observed in the output of single-fre-quench solid-state lasers. For example, such instable-ties are discussed by Daniel Meyer, IEEE J. Quantum Electronics, Vol. QUEUE, No. 2, February 1970, pp. 101-104.
Summary of the Invention The present invention is directed to a method for improving the amplitude stability of a solid-state laser.
A high degree of amplitude stability is desirable if the output radiation from such a laser is to be used in apply-cations such as optical storage of data, spectroscopy, communications, sensor systems, projection displays, laser printing and laser film read/write systems.
A solid-state laser which typically emits output radiation of a single-frequency can be obtained by elm-noting spatial hole burning in the lasant material. We have found that the amplitude fluctuations (noise) in the I I
output radiation from such a device are a function of them-portray and drop to very low values over certain well-de-fined temperature ranges or "temperature windows."
Accordingly, we have discovered that substantially noise-S free operation can be achieved by maintaining the temper-azure of the laser cavity at a value within such a temperature window.
One embodiment of the invention is a method for gent crating coherent optical radiation which comprises:
(a) generating coherent optical radiation from solid lasant material within an optical cavity for said radix anion; (b) substantially preventing spatial hole burning in the lasant material during the generation of said radix anion with spatial hole burning suppression means;
(c) withdrawing at least a portion of said radiation from the optical cavity as output; and (d) maintaining the them-portray of the optical cavity within a range over which said output is substantially noise-free.
Another embodiment of the invention is an optically-pumped solid-state laser comprising: (a) a standing wave optical cavity; (b) optical pumping means for generating optical pumping radiation, wherein said optical pumping means comprises a laser diode; (c) solid lasant material which is disposed within said optical cavity, positioned to receive pumping radiation from said optical pumping means, and effective for generating coherent optical radix anion upon receiving the pumping radiation; (d) means log substantially eliminating spatial hole burning in the lasant material during said generation of coherent optical radiation; and (e) temperature control means for maintain-in the temperature of the optical cavity at a value which results in substantially noise free generation of said coherent radiation.
An object of the invention is to provide a laser diode-pumped solid state laser of improved amplitude stay ability.
2~2~
Another object of the invention is to provide a laser diode-pumped, twisted-mode, solid-state laser of improved amplitude stability.
A further object of the invention is to provide a method for reducing the amplitude instabilities that are observed in the output of a solid-state laser.
A still further object of the invention is to provide a method for improving the amplitude stability of a laser diode-pumped, twisted-mode, solid-state laser.
Brief Description of the Draw nags FIG. 1 of the drawings is a schematic illustration of one embodiment of the invention.
FIG. 2 of the drawings illustrates the optical output power of a laser diode-pumped solid-state laser of the type illustrated by FIG. 1 and the root mean square (RUMS) noise content of said output as a function of the temper-azure of the laser cavity.
FIG. 3 of the drawings is a schematic illustration of a second embodiment of the invention.
Detailed Description of the Preferred Embodiments While this invention is susceptible of embodiment in many forms, two specific embodiments are schematically shown in FIGS. 1 and 3 of the drawings, with the under-standing that the present disclosure is not intended to limit the invention to the embodiments illustrated.
With reference to FIG. 1, optical pumping radiation 1 from optical pumping means 2 and 3 is focused by focusing means 4 through quarter-wave plate 5 and into solid lasant material 6 which is capable of being pumped by the radix anion from said pumping means (2 and 3). Light emitted by the losing of lasant material 6 oscillates within the linear standing wave optical cavity defined by mirror 7 and a suitable reflective coating on surface 8 of quart 2~2~
I
ter-wave plate 5, and such light is hereinafter referred to as cavity radiation. A second quarter-wave plate 9 and a Brewster plate polarizer 10 are positioned between lasant material 6 and mirror 7. The pair of quarter-wave plates 5 and 9 serves to suppress spatial hole burning within lasant material 6, and the Brewster plate polarizer 10 causes the cavity radiation to be linearly polarized at mirror 7 and also determines the direction of polarize-lion. A portion of the cavity radiation is passed through mirror 7 as output radiation 11. Finally, the laser illustrated in FIG. 1 is provided with temperature control means which is not shown in the figure) for adjusting and controlling the temperature of the laser's optical cavity and its contents.
Optical pumping means 2 and 3 can comprise any con-ventional source of optical pumping radiation. However, preferred sources of pumping radiation 1 include light-em-witting diodes and laser diodes. Such diodes are commonly attached to a thermally conductive heat sink and are pack-aged in a metal housing. For efficient operation, the pumping radiation 1 is matched with a suitable absorption band of the lasant material 6. Conventional light-emit-tying diodes and laser diodes are available which, as a function of composition, produce output radiation having a wavelength over the range from about 630 no to about 1600 no, and any such device producing pumping radiation 1 of a wavelength effective to pump lasant material 6 can be used in the practice of this invention. For example, the wavelength of the output radiation from a Gain based device can be varied from about 630 to about 700 no by variation of the device composition. Similarly, the wave-length of the output radiation from a Galas based device can be varied from about 750 to about 900 no by variation of the device composition, and InGaAsP based devices can be used to provide radiation in the wavelength range from about 1000 to about 1600 no.
A highly suitable source of optical pumping radiation 1 consists of a gallium aluminum arsenide laser diode array 3, emitting light having a wavelength of about 810 no, which is attached to heat sink 2. Heat sink 2 can be passive in character. However, heat sink 2 can also comprise a thermoelectric cooler to help maintain laser diode array 3 at a constant temperature and thereby ensure optimal operation of laser diode array 3 at a constant wavelength. It will be appreciated, of course, that during operation the optical pumping means will be attached to a suitable power supply. Electrical leads from laser diode array 3 which are directed to a power supply are not illustrated in FIG. 1.
Focusing means 4 serves to focus pumping radiation 1 through quarter-wave plate 5 and into lasant material 6.
This focusing results in a high pumping intensity and an associated high photon to photon conversion efficiency in lasant material 6. hocusing means 4 can comprise any con-ventional means for focusing light such as a gradient index lens, a ball lens, an spheric lens or a combination of lenses.
Any conventional solid lasant material 6 can be Utah-lived provided that it is capable of being optically pumped by the optical pumping means selected. Suitable lasant materials include, but are not limited to, solids selected from the group consisting of glassy and crystal-line host materials which are doped with an active mate-fiat and substances wherein the active material is a stoichiometric component of the lasant material. Highly suitable active materials include, but are not limited to, ions of chromium, titanium and the rare earth metals.
Highly suitable lasant materials include neodymium-doped YAW, neodymium-doped YALE, neodymium-doped ELF, neodymi-um-doped GSGG, neodymium pentaphosphate and lithium needy-mum tetraphosphate. my way of specific example,neodymium-doped YAW is a highly suitable lasant material 6 for use in combination with an optical pumping means which produces light having a wavelength of about 810 no. When pumped with light of this wavelength, neodymium-doped YAW
can emit light having a wavelength of 1064 no.
The precise geometric shape of lasant material 6 can vary widely. For example, lasant material 6 can be rod-shaped, or rhombohedral in shape if desired, and lens-shaped surfaces can be used if desired. If desired, an end-pumped fiber of lasant material can be used. Highly suitable fibers for this purpose include, but are not limp tied to, glass optical fibers which are doped with ions ova rare earth metal such as neodymium. The length of such a fiber is easily adjusted to result in absorption of essentially all of the optical pumping radiation 1. If a very long fiber is required, it can be coiled, on a spool for example, in order to minimize the overall length of the laser of this invention.
The reflective coating on surface 8 of quarter-wave plate 5 is selected in such a manner that it is sub Stan-tidally transparent to optical pumping radiation 1 but highly reflective with respect to the cavity radiation produced by the losing of lasant material 6.
Mirror 7 is selected in such a manner that it is par-tidally transmitting for the cavity radiation produced by the losing of lasant material 6. Mirror 7 is conventional in character and, for example, can comprise any suitable conventional coating on any suitable substrate.
The pair of quarter-wave plates 5 and 9 are quarter-wave plates for cavity radiation and serve as a means for substantially eliminating spatial hole burning by causing circular polarization of the cavity radiation within lasant material 6 [this technique for producing an axially uniform energy density in a lasant material is described by V. Evtuhov et at., Apply Optics, Vowel, Noel, pp. 142-143 (1965)]. The precise location of these two quarter-wave plates within the optical cavity is not critical provided that lasant material 6 is placed between them.
Brewster plate polarizer 10 results in the linear polarization of cavity radiation at mirror 7 and also determines the direction of polarization. This polarize-lion permits convenient optimization of the orientation of quarter-wave plates 5 and 9 for elimination of spatial hole burning in lasant material 6. However, Brewster plate polarizer 10 is not an essential element of the invention and is not required to effect substantial elm-nation of spatial hole burning with quarter-wave plates 5 and 9.
A highly satisfactory orientation for quarter-wave plates 5 and 9 and for Brewster plate polarizer 10 involves positioning the quarter-wave plates so that the optic axis of one is substantially perpendicular with respect to the optic axis of the other about the axis along which they encounter cavity radiation and position-in Brewster plate polarizer 10 so that its highly trays-milting axis is oriented at an angle of about 45 with respect to the optic axis of each quarter-wave plate.
Any conventional means for substantially eliminating spatial hole burning in the lasant material can be used in the practice of this invention. For example, spatial hole burning can be eliminated through the use of a traveling-wave ring-like optical cavity (which is illustrated by the embodiment set forth in FUGUE), by generating circularly polarized light in the lasant material (which is thus-treated in FIG. 1 and is effected by the pair of quarter-wave plates 5 and 9), with mechanical motion, or with electro-optic phase modulation.
The laser illustrated by FIG. 1 is provided with them-portray control means for adjusting and controlling the temperature of the laser's optical cavity and its con-tents. This temperature control means can be of any con-ventional type, for example, an electrically powered resistance heater or thermoelectric device, and is used to maintain the temperature of the optical cavity at a value which results in substantially noise-free generation of 2~2~
output radiation 11. Through the use of such temperature control means, the temperature of the cavity and its con-tents is desirably maintained at `1C of the selected value, preferably at `0.5C of the selected value and more preferably at `0.1C of the selected value.
The undesired fluctuations in the amplitude of output radiation 11, for example, within a frequency range of about 1 kHz to about 50 MHz, are referred to as noise and are conveniently measured as percent root mean square (%
RUMS) noise. Although a laser of improved amplitude stay ability is obtained through the substantial elimination of spatial hole burning in the lasant material 6, we have found that the noise content of output radiation 11 drops essentially to zero over certain temperature ranges. More specifically, if the noise content of output radiation 11 is measured as a function of the temperature of the laser cavity, certain ranges of temperature or "temperature win-dowse are observed over which the noise drops to Essex-tidally zero. The width of these windows and the precise temperatures at which they occur are different for each individual laser. That is to say, we have found that the noise content of output radiation if as a function of the temperature of the laser cavity is a unique characteristic of each device. However, for a given laser, the noise content of the output radiation 11 as a function of them-portray does not change significantly over long periods of time (for example, weeks or months) or with repeated cycling over large temperature ranges. Accordingly, a highly preferred embodiment of this invention comprises locating a window of substantially noise-free operation for a laser by measuring the noise content of output radix anion 11 as a function of the temperature of the optical cavity and maintaining the temperature of the laser cavity at a value within such a window during subsequent opera-lion. These windows of substantially noise-free operation are typically from about 1 to about 15C wide, and within such a window the RUMS noise will typically be less than about 0.2% and frequently less than about 0.1%. The win-dowse of substantially noise-free operation are easily identified by measuring the noise content of output radix anion 11 over any range of temperature which is convenient from an operating point of view, for example, from about 0 to about 100C, or more conveniently, from about 30 to about 65C. This measurement is desirably carried out over a range of at least about 5C and preferably over a range of at least about 10C or 20C in order to give a reasonable sampling of the laser's temperature-related performance.
In a specific example of the embodiment illustrated in FIG. 1, neodymium-doped YAW is used as lasant material 6. The neodymium-doped YAW is optically pumped by a mull tistripe laser diode array 3 which is attached to a then-moelectric cooler 2 (the array and attached thermoelectric cooler is a Model SOL 2422-Hl device manufactured by Specs ire Diode Labs of San Jose, California). The laser diode array 3 is a 10-stripe array consisting of 3 micron stripes on 10 micron centers which can provide about 200 my of pumping radiation 1 having a wavelength of about 810 no. This pumping radiation 1 is focused by gradient index lens 4 which has a 0.29 pitch and is anti reflection coated with respect to 810 no wavelength radiation. The focused pumping radiation passes through quarter-wave retardation plate 5 which is comprised of quartz and is in the form of a circular plate having a thickness of about 1 mm and a 10 mm diameter. Input face 8 of quarter-wave plate 5 carries a multi layer dielectric coating which is highly reflective (R > 99.8~) at a wavelength of 1064 no and highly trays-parent (T > 80%) at a wavelength of 810 no. Output face 12 of ~uarter-wave plate 5 carries an anti reflection coat-in (R < 0.2~) for light having a wavelength of 1064 no and highly transparent (T > 80%) at a wavelength of 810 no. The focused pumping radiation comes to a focus within lasant material 6 which contains about I neodymium and is in the form of a disc having a 4 mm thickness and a 10 mm diameter. The lasant material 6 is oriented for low threshold operation at a wavelength of 1064 no and emits light (cavity radiation) having a wavelength of 1064 no in response to excitation by the pumping radiation. The sun-faces of lasant material 6 are anti reflection coated (R <
0.2%) for light having a wavelength of 1064 no and highly transparent (T > 80%) at a wavelength of 810 no. Quart ter-wave plate 9 is identical with plate 5 except that both of its surfaces are anti reflection coated with respect to 1064 no wavelength radiation. Quarter-wave plates 5 and 9 are positioned in such a manner that the optic axis of one makes a 90 angle with respect to the optic axis of the other about the axis along which they encounter cavity radiation. The highly transmitting axis lo of Brewster plate polarizer 10 is oriented so that it makes an angle of 45 with respect to the optic axis of each of the quarter-wave plates 5 and 9 about the axis along which these components encounter cavity radiation.
Output radiation 11 having a frequency of 1064 no is tray-spitted through mirror 7 which has a radius of curvature of 30 cm and carries a dielectric coating which is 97%
reflective at a wavelength of 1064 no. The optical cavity of this laser has a length (distance from surface 8 to mirror 7) of about 20 mm. The optical cavity is wrapped I with an electrically powered resistance heater and fitted with a thermistor which can be used to: (a) measure the cavity temperature; and (b) control the cavity temperature by providing a feedback signal to the power supply for the resistance heater.
The power and percent root mean square (RUMS) noise of the 1064 no output radiation from the above-described laser were measured a a function of temperature over the range from about 40 to about 60C. The results are set forth in FIG. 2. With reference to FIX. I, it will be noted that the RUMS noise in the output radiation drops to very low values over certain ranges of temperature or "temperature windows." For example, such a window for single frequency operation appears at about 40.0~43.0C.
The precise location and width of these windows of sub-staunchly noise-free operation are unique characteristics of each individual laser and do not change significantly with time (for example, weeks or months) or repeated them-portray cycling over several tens of degrees centigrade.
Accordingly, each laser can be made to operate in a sub-staunchly noise-free manner by maintaining the optical cavity of the device at a temperature within such a window during operation.
It will also be noted from FIG. 2 that the laser functions in a multi longitudinal mode manner (multimedia operation) over certain temperature ranges. This is typic eel behavior for a solid-state laser in which spatial hole burning has been suppressed, and a preferred embodiment of the invention involves selecting a temperature window which yields: (a) substantially noise-free operation, and (b) single transverse mode operation.
FIG. 3 schematically illustrates a second embodiment of the invention which involves the use of a traveling-wave ring-like optical cavity for the purpose of sub Stan-tidally eliminating spatial hole burning in the lasant material. With reference to FIG. 3, optical pumping radix anion 21 from laser diode array 22 is focused by focusing means 23 into lasant material 24 which is capable of being pumped by said pumping radiation. Laser diode array 22 is attached to heat sink 25. Light emitted by the losing of lasant material 24 is contained within the optical cavity defined by mirrors 26, 27 and 28 and by a suitable reflect live coating on surface 29 of lasant material 24, and sociality is hereinafter referred to as cavity radiation. A
unidirectional optical gate means for effecting unidirec-tonal circulation of cavity radiation within the optical cavity is provided by the combination of polarizer 30, Faraday rotator 31, and half-wave plate 32. A portion of the cavity radiation is passed through mirror I as output radiation 33. Finally, the laser illustrated in FIG. 3 is provided with temperature control means (which is not shown in FIG. 3) for adjusting and controlling the temper-azure of the optical cavity and its contents. This them-portray control means is employed to maintain the optical cavity of the laser at a value which results in sub Stan tidally noise-free generation of output radiation 33.
The reflective coating on surface 29 of lasant mate-fiat 24 is selected in such a manner that it is sub Stan-tidally transparent to optical pumping radiation 21 but highly reflective with respect to the cavity radiation produced by the losing of lasant material 24. Mirrors 26 and 27 are highly reflective for the cavity radiation pro-duped by the losing of lasant material I Mirror 28 is partially transmitting for cavity radiation.
Any conventional polarization means can be utilized as polarizer 30, for example, a Brewster plate, suitable coatings on the mirrors of the optical cavity, a Delco-trig polarizer, or a Brewster angle surface on the lasant material 24.
If neodymium-doped YAW is used as the lasant material 24, the YAW crystal itself can also serve as Faraday rota-ion 31 if a magnetic field is established along the axis of the crystal. In such an embodiment, a separate Faraday rotator 31 is not required.
Conventional designs for a traveling-wave optical cavity which can be employed in the practice of this invention for the purpose of substantially eliminating spatial hole burning in the last material are set forth in W. Cautionary, Solid-State Laser Engineering (Springer-30 Verlag, New York, Second Ed., 1988) at pp. 126-128 and in Siegman, Lasers (University Science Books, Mill Valley, California, 1986) at pp. 532-538.
Claims (20)
1. A method for generating coherent optical radi-ation which comprises:
(a) generating coherent optical radiation from solid lasant material within an optical cavity for said radiation;
(b) substantially preventing spatial hole burn-ing in the lasant material during the generation of said radiation with spatial hole burning suppression means;
(c) withdrawing at least a portion of said radiation from the optical cavity as output; and (d) measuring the noise in said output as a function of the temperature of the optical cavity;
(e) determining a first temperature range for the optical cavity over which said output is substan-tially noise-free; and (f) maintaining the temperature of the optical cavity at a value which is within said first temper-ature range during subsequent production of said output.
(a) generating coherent optical radiation from solid lasant material within an optical cavity for said radiation;
(b) substantially preventing spatial hole burn-ing in the lasant material during the generation of said radiation with spatial hole burning suppression means;
(c) withdrawing at least a portion of said radiation from the optical cavity as output; and (d) measuring the noise in said output as a function of the temperature of the optical cavity;
(e) determining a first temperature range for the optical cavity over which said output is substan-tially noise-free; and (f) maintaining the temperature of the optical cavity at a value which is within said first temper-ature range during subsequent production of said output.
2. The method of claim 1 which additionally com-prises:
(a) measuring the longitudinal mode character-istics of said output as a function of the temper-ature of the optical cavity;
(b) determining a second temperature range for the optical cavity over which said output is single-frequency and wherein said second temperature range overlaps said first temperature range; and (c) maintaining the temperature of the optical cavity at a value which is within said first and second temperature ranges during subsequent pro-duction of said output.
(a) measuring the longitudinal mode character-istics of said output as a function of the temper-ature of the optical cavity;
(b) determining a second temperature range for the optical cavity over which said output is single-frequency and wherein said second temperature range overlaps said first temperature range; and (c) maintaining the temperature of the optical cavity at a value which is within said first and second temperature ranges during subsequent pro-duction of said output.
3. The method of claim 1 which additionally com-prises:
(a) measuring the noise in said output as a function of the temperature of the optical cavity;
(b) determining a temperature range for said optical cavity over which said output is substan-tially noise-free; and (c) maintaining the temperature of the optical cavity at a value which is within said determined temperature range during subsequent production of said output.
(a) measuring the noise in said output as a function of the temperature of the optical cavity;
(b) determining a temperature range for said optical cavity over which said output is substan-tially noise-free; and (c) maintaining the temperature of the optical cavity at a value which is within said determined temperature range during subsequent production of said output.
4. The method of claim 1 wherein the optical cavity is a standing wave cavity for said radiation.
5. The method of claim 4 wherein the optical cavity is a linear standing wave cavity for said radiation.
6. The method of claim 4 wherein said spatial hole burning suppression means comprises a pair of quarter-wave plates for effecting circular polarization of said radi-ation in the lasant material.
7. The method of claim 6 wherein the optical cavity contains a Brewster plate polarizer.
8. The method of claim 7 wherein the optic axis of one quarter-wave plate is substantially perpendicular to that of the other quarter-wave plate, and the highly tran-smitting axis of said Brewster plate polarizer is oriented at an angle of about 45° with respect to the optic axis of each quarter-wave plate.
9. The method of claim 1 wherein said optical cavity is a ring-type cavity and said spatial hole burning suppression means comprises unidirectional optical gate means for effecting unidirectional circulation of said radiation within the optical cavity.
10. The method of claim 9 wherein said unidirec-tional optical gate means is comprised of a polarizer, a half-wave plate, and a Faraday rotator.
11. The method of claim 1 which additionally com-prises optically-pumping said lasant material with optical pumping means wherein said optical pumping means comprises a laser diode.
12. The method of claim 11 wherein said optical pumping means comprises a laser diode array.
13. An optically-pumped solid-state laser compris-ing:
(a) a standing wave optical cavity;
(b) optical pumping means for generating optical pumping radiation, wherein said optical pumping means comprises a laser diode;
(c) solid lasant material which is disposed within said optical cavity, positioned to receive pumping radiation from said optical pumping means, and effective for generating coherent optical radi-ation upon receiving the pumping radiation;
(d) means for substantially eliminating spatial hole burning in the lasant material during said generation of coherent optical radiation; and (e) temperature control means for maintaining the temperature of the optical cavity within a temperature range which results in substantially noise-free generation of said coherent radiation wherein said temperature range is determined by evaluating the noise in said coherent radiation as a function of the temperature of said optical cavity.
(a) a standing wave optical cavity;
(b) optical pumping means for generating optical pumping radiation, wherein said optical pumping means comprises a laser diode;
(c) solid lasant material which is disposed within said optical cavity, positioned to receive pumping radiation from said optical pumping means, and effective for generating coherent optical radi-ation upon receiving the pumping radiation;
(d) means for substantially eliminating spatial hole burning in the lasant material during said generation of coherent optical radiation; and (e) temperature control means for maintaining the temperature of the optical cavity within a temperature range which results in substantially noise-free generation of said coherent radiation wherein said temperature range is determined by evaluating the noise in said coherent radiation as a function of the temperature of said optical cavity.
14. The laser of claim 13 wherein the optical cavity is a linear standing wave cavity.
15. The laser of claim 13 wherein said temperature control means is effective to maintain said optical cavity at a substantially constant temperature.
16. The laser of claim 13 wherein said optical pump-ing means comprises a laser diode array.
17. The laser of claim 13 wherein said lasant mate-rial is comprised of neodymium-doped YAG.
18. The laser of claim 13 wherein said means for substantially eliminating spatial hole burning comprises a pair of quarter-wave plates within the optical cavity, and said lasant material is positioned between the quarter-wave plates.
19. The laser of claim 18 wherein the optical cavity contains a Brewster plate polarizer.
20. The laser of claim 19 wherein the optic axis of one quarter-wave plate is substantially perpendicular to that of the other quarter-wave plate, and the highly tran-smitting axis of said Brewster plate polarizer is oriented at an angle of about 45° with respect to the optic axis of each quarter-wave plate.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/353,870 US4933947A (en) | 1988-02-18 | 1989-05-18 | Frequency conversion of optical radiation |
| US415,641 | 1989-10-02 | ||
| US353,870 | 1989-10-02 | ||
| US07/415,641 US5031182A (en) | 1989-05-18 | 1989-10-02 | Single-frequency laser of improved amplitude stability |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2026146A1 true CA2026146A1 (en) | 1991-04-03 |
Family
ID=26998124
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2026146 Abandoned CA2026146A1 (en) | 1989-05-18 | 1990-09-25 | Single-frequency laser of improved amplitude stability |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2026146A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112952534A (en) * | 2021-01-27 | 2021-06-11 | 山西大学 | Device for suppressing laser intensity noise by using frequency doubling cavity and measuring method thereof |
-
1990
- 1990-09-25 CA CA 2026146 patent/CA2026146A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112952534A (en) * | 2021-01-27 | 2021-06-11 | 山西大学 | Device for suppressing laser intensity noise by using frequency doubling cavity and measuring method thereof |
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