CA2141599A1 - Depolarized solid-state diode-pumped laser - Google Patents
Depolarized solid-state diode-pumped laserInfo
- Publication number
- CA2141599A1 CA2141599A1 CA 2141599 CA2141599A CA2141599A1 CA 2141599 A1 CA2141599 A1 CA 2141599A1 CA 2141599 CA2141599 CA 2141599 CA 2141599 A CA2141599 A CA 2141599A CA 2141599 A1 CA2141599 A1 CA 2141599A1
- Authority
- CA
- Canada
- Prior art keywords
- laser
- optical
- lasant material
- face
- light
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- 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
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/08018—Mode suppression
- H01S3/08022—Longitudinal modes
- H01S3/08031—Single-mode emission
- H01S3/08036—Single-mode emission using intracavity dispersive, polarising or birefringent elements
-
- 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
- H01S2302/00—Amplification / lasing wavelength
-
- 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
- H01S3/09—Processes or apparatus for excitation, e.g. pumping
- H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
- H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
- H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
Abstract
2141599 9428603 PCTABS00034 The disclosed invention, a laser and method for producing depolarized laser light, pertains to the field of laser radiation production. One embodiment of the laser comprises a solid state non-birefringent lasant material (18), a laser diode source (S) and birefringent optical retarding means (24) for transmitting light from the laser diode source to the lasant material, for reflecting laser light produced by the lasant material towards the output coupler (20) to form an optical cavity for the lasant material, and for forming in the cavity single line multimode laser light having at least two longitudinal standing laser modes characterized by two closely spaced apart orthogonal linear polarizations of approximately the same power level.
Description
WO 94/28603 ~ l 41 ~s 9 9 PCTII~S94/061Bl T~chni~l Fi~ld - 7his invention relates to ~he general subject of lasers and, in particular to diode-pumped, solid-state lasers and methods for producing a depolarized output.
Baçk~,ourld ~f th~Inve,ntion Fiberoptic sensors using Mach-Zehnder modulators often require a stable, polarized optical carrier at the output end of a long length of 1 Q variable birefringence optical fiber. This occurs in coherent cornmunications and in fiber sensors to avoid input lead pol~iza~ion noise and polarization fading. In general, ~ree approaches have been employed to solve this problem: using polarization preserving fiber; active polarization control with a birefringence device which produces the 1~ desired state at ~e ou~put in conjunction widl a feedback loop; and using depolarized light which will always remain depolarized independent of fiber birefringence. (See A.D. Kersey, et al., Electron Lett., 23; p. 634 and p. 924; 1987).
The second method is illus~rated by the system developed by 20 Marshall, at EG&G Energy Measurements. That system uses discre$e retardation plates located in fron~ of the laser. A computer monitors the feedback signal and uses! stepper motor rotation stages to adjust the launch polariza~ion state.
The third approach has the advantage of avoiding expensive 25 polariza~ion preserving fiber and does not require complex electronic feedback control loops. One convenient way of achieving this is by combining a depolarized optical source with a polarizer at the modulator .~.~
. ~
WO 94/28603 ;~14 1 ~i 3 ~ PCT/US94/061~1 input. This approach has been demons~ated using two polarization-couple~, frequency-o~fset, single-~requency lasers as a source. (See W.R.
Burns, R.P. Moeller, (:~.H. Bulmer and A.S. Greenblatt, "Depolarized source for fiber-optic applications"; ~Lett., 16(6), 381 ~1991) and B.
5 Marshall "PolaAzation Control Systems" Proceedin~s PSAA-91, pp. 83 (1~91))-In particular, the system of Burns et al. used ring cavity,diode-pumped Nd:Y~G lasers which have precise narrow linewidths ~at can be thermally tuned. These lasers are normally linearly polarized.
10 The depolanzed source was simply created by combining the beams from two lasers whose linear polarization states are at 90 to each other, and adjus~ing the thermal controllers so ~at ~e frequency difference bet~Neen the two sources is a~ some large frequency, compared to ~e detection bandwid~ of the system of interest.
All of these methods are relatively complicated, and in many applications a simpler, single-laser, depolarized source would be desirable.
A laser with a polarization ratio near unity and a mode spacing large enough to keep intermodulation products out of the signal band is 20 preferred. Multi-longitudinal mode operation would be acceptable, provided that the overall linewidth is small enough to avoid polarization dispersion effects in the fiber. Although a linear cavity Nd:YAG laser with no intracavity polarizers passively satisfies the polarization requirement, such a randomly polarized laser is not directly useful 2~ because frequency splittings of a few MHz between nominally degenerate modes are often introduced by stresses in the laser rod. Therefore, there is still room for improvement.
--; 2 , i .~ .
Baçk~,ourld ~f th~Inve,ntion Fiberoptic sensors using Mach-Zehnder modulators often require a stable, polarized optical carrier at the output end of a long length of 1 Q variable birefringence optical fiber. This occurs in coherent cornmunications and in fiber sensors to avoid input lead pol~iza~ion noise and polarization fading. In general, ~ree approaches have been employed to solve this problem: using polarization preserving fiber; active polarization control with a birefringence device which produces the 1~ desired state at ~e ou~put in conjunction widl a feedback loop; and using depolarized light which will always remain depolarized independent of fiber birefringence. (See A.D. Kersey, et al., Electron Lett., 23; p. 634 and p. 924; 1987).
The second method is illus~rated by the system developed by 20 Marshall, at EG&G Energy Measurements. That system uses discre$e retardation plates located in fron~ of the laser. A computer monitors the feedback signal and uses! stepper motor rotation stages to adjust the launch polariza~ion state.
The third approach has the advantage of avoiding expensive 25 polariza~ion preserving fiber and does not require complex electronic feedback control loops. One convenient way of achieving this is by combining a depolarized optical source with a polarizer at the modulator .~.~
. ~
WO 94/28603 ;~14 1 ~i 3 ~ PCT/US94/061~1 input. This approach has been demons~ated using two polarization-couple~, frequency-o~fset, single-~requency lasers as a source. (See W.R.
Burns, R.P. Moeller, (:~.H. Bulmer and A.S. Greenblatt, "Depolarized source for fiber-optic applications"; ~Lett., 16(6), 381 ~1991) and B.
5 Marshall "PolaAzation Control Systems" Proceedin~s PSAA-91, pp. 83 (1~91))-In particular, the system of Burns et al. used ring cavity,diode-pumped Nd:Y~G lasers which have precise narrow linewidths ~at can be thermally tuned. These lasers are normally linearly polarized.
10 The depolanzed source was simply created by combining the beams from two lasers whose linear polarization states are at 90 to each other, and adjus~ing the thermal controllers so ~at ~e frequency difference bet~Neen the two sources is a~ some large frequency, compared to ~e detection bandwid~ of the system of interest.
All of these methods are relatively complicated, and in many applications a simpler, single-laser, depolarized source would be desirable.
A laser with a polarization ratio near unity and a mode spacing large enough to keep intermodulation products out of the signal band is 20 preferred. Multi-longitudinal mode operation would be acceptable, provided that the overall linewidth is small enough to avoid polarization dispersion effects in the fiber. Although a linear cavity Nd:YAG laser with no intracavity polarizers passively satisfies the polarization requirement, such a randomly polarized laser is not directly useful 2~ because frequency splittings of a few MHz between nominally degenerate modes are often introduced by stresses in the laser rod. Therefore, there is still room for improvement.
--; 2 , i .~ .
3 2 1 `I 1 i 3 9 PCT/US94/061Bl A general object of the invention is to provide a diode-pumped, solid-state laser ha ring a depolarized output.
Yet ano~er object of the invention is to provide a polarized op~ical 5 carrier for a communications system utili~ing an extemal optical modulator and a non-polarization maintaining optical fiber.
Still ano~er object of ~e invention is to proYide an apparatus and me~od for producing laser light cha~acterized by at least two longitudinal standing modes having two or~ogonal linear polarizations of about the 10 same power level.
One specific object of the ~nvention is to provide a Nd:YAG laser having a quarterwave plate to produce two polanzation eigenstates of substantially equal intensity and a single line ou~put.
In accordance with one embodiment of ~e presen~ invention, a 15 laser apparatus is provided comprising: a solid-state minimally bir~fringent lasant material having an input face and an opposite ou~ut face; an output coupler disposed towards said output face; pumping means for pumping light into said input face of said lasant material to pr~duce a population inversion therein; optical retardin~ means, having 20 one face abutting said input face of said lasant material and having an opposi~e face coated for transmitting light from said pumping means to said lasant material and for reflecting laser light produced by said rnaterial towards said output coupler to form an optical cavity for said - lasant material, for forming in said cavity single line multimode laser 25 light having at least two longitudinal standing laser modes characterized - by two closely spaced apart orthogonal linear polarizations of approximately the same power level; and means for avoiding high order transverse laser modes in said cavity.
: 3 .~
t WO ~4/28603 PCT/US94/06181 ~1~159~
In addition a method is disclosed for providing a polarized optical ca~Tier for use in a commlmications system of ~he type using an extemal modulator which operates in response to an optical calTier supplied through an optical fiber, comprising ~he steps of: forming an optical cavity ~or a lasant material having an input face and an opposite ou~ut face by iocating an oucput coupler disposed towards said output face and by locating a quarte~wave plate having one face abutting said input face and having an opposite face that is coated ~or transmitting optical pumping light to said lasant material and for reflecting laser light 0 produced by said lasant material towards said ou~Lpu~ coupler, plroviding a plurality of semi-conductors that, in response to the flow of electrical current therethrough, produces optical pumping light; purnping said lasant material using said semi-conductors to produce a population ir:version in - said lasant material; substantially reducing high order transverse modes in 15 said cavi~y; and adjusting said optical cavity to have an optical leng~ ~at is propo~ nal to one-half the longitudinal mode spacing of said laser light and to produce standing wave, substantially single frequency laser light that is characterized by two closely spaced apart orthogonal linear polarizations of about the same power level out of said output coupler for 20 at least some power levels of said semi-conductors.
Other advantages and feanlres of the present invention will become readily apparent from the following detailed description of the invention, the embodiments described therein, from the claims, and from the accompanying drawings.
2~
Brief De~ription of the.Drawings FIG. 1 is a schematic diagram of the laser that is the subject of the present invention; and :-~ 4 .
~ ~ 4~ ~ 99 ~;IG. 2 is depicts the perfo~nance of ~e laser illus~ated in FIG. 1.
While ~is invention is susceptible to embodiment in many different ~orms, there is shown in the drawings, and will herein be described in - detail, one speci~lc embodiment of ~e invencion. It should be understood, however, dlat ~e present disclosure is to be considered an e~emplification of ~e principles of the invention and is not intended to limit ~e invention to dle specific embodiment illustrated.
It is to be understood that the terms "light", "light pulse" and "laser energy" are used in this application not as limited to the visible spectrum of electromagnetic radiation~ but as in the broad sense of "radiant energy". :
Turning to FIG. 1, dlere is illustrated a diode-pumped solid-state laser 10, a non-polarization main~aining optical fiber 12, a polarizer 14 and an optical modulator 16. The laser 10 comprises a solid-state non-birefringent lasant material 18, an oucput coupler 20, an aperture plate 22, a quarterwave plate 24, and an input mirror 26. The lasant material 18 is optically pumped by a source S. `
Suitable optical pumiping means or sources S include, but are not limited to9 laser diodes, light-emitting diodes (including superluminescent diodes and superluminescent diode arrays~ and laser diode arrays, ` ` . ! i together with any ancillary packaging or structures. For the purposes hereof, the teIm "optical pumping means" includes any heat sink, 2~ thermoelectric cooler or packaging associated with said laser diodes, light-emitting diodes and laser diode arrays. For example, such devices are commonly attached to a heat resistant and conductive heat sink and a~e paclcaged in a metal housing. For efficient operation, ~e pumping means ~:~
; ~
S is desirably matched with a suitable absorption band of the lasant material 18. Although the invention is not to be so limited, a highly suitable optical pumping souree consists of a gallium aluminum arsenide laser diode, which emits light having a wavelength of about 810 nm, that is attached to heat sink. Heat sink can be passive in character. However, heat sink can also compromise a thermoelectric cooler or other tempera~re regulation means to help main~ain laser diode at a constant temperaturè and thereby ensure optimal operation of laser diode at a constant wavelength. It will be appreciated, of course, ~lat during operation the optical pumping means will be attached to a suitable power supply. Electrical leads from laser diode, which are directed to a suitable power supplyy are not illus~ated in dle drawings.
Conventional light-emitting diodes and laser diodes are available which, as a function of composition, produce output radiation having a 1~ waveleng~ over the range from about 630 nrn to about 1600 nm. Any device producing optical pumping radiation of a wavelength effective to pump a lasant material 18 can be used in the practice of this invention.
Por example, the waveleng~ of the output radiation from a GaInP based device can be varied from about 630 to about 700 nm by variation of the device composition. Similarly, the wavelength of the output radiation from a GaAlAs based device can be varied from about 750 to about 900 nm by variation of the device composition. InGaAsP based devices can be , used to provide radiation in the wavelength range from about lO00 to about 1600 nm.
If desired, the output facet of semiconductor light source S can be placed in butt-coupled relationship to input surface of the lasant material wi~out the use of inteImediate optics. As used herein, "butt-coupled" is defined to mean a coupling which is sufficiently close such that a "
.~
214~599 divergent beam of op~ieal pumping r3diation emanating from semiconductor light source or laser diode S will optically pump a mode volume within the lasant material 18 wi~ a sufficiently small transverse cross-xectional area so as to support essentially only single transverse 5 mode laser operation (i.e., TEMoo mode operation) in ~e lasant material.
Suitable lasant materials 18 include, but are not limited to, solids selected ~rom ~e group consis~ng of glassy and crystalline host materials which are ~oped with an active material and substances wherein ~e active material is a s~oichiometrie component of the lasant material. By way of 10 specific example, neodymium-doped YAG or Nd:YAG is a highly suitable lasant material for use in combination with an optical pllIIlpiIlg means S
which produces ligh~ having a waveleng~ of about 808 nun. When pumped with light of ~is waveleng~, neodymium-doped YAG can emit light having a wavelength of either about 1064 nm or about 1319 nm.
1~ Nd YAG is a non-birefringent lasant material. Preferably the lasant material îs "non-birefringent". By non-birefringent is meant ~at the gain medium has minimal birefringence in the direction of laser propagation.
Thus, a birefringent material like Nd:YLF can be used provided that ~e laser propagates along an optic axis of the crystal where it is "effectively"
20 nonbirefringent.
The lasant material 18 is located in an optical cavity which is fo~ ed by the output coupler 20 and the input mirror 26. l~e lasant method has two opposite ends 18a and 18b. One end 18a faces the output coupler 20 and the opposite end 18b faces the input mirror 26. When 2~ pumped by the source S, a population inversion is produced and the lasant material produces laser light.
The quarterwave plate 24 functions as a birefringent optical retarding means. It has one face 24a which abuts the input face 18b of dle WO 94/28603 PCT/VSg4/06181 ~141a99 lasant material 18, and it has an opposite or outside face 24b which receives pumping light from ~he source S. In one embodiment, the outside ~ace 24b is coa~d to function as the ~put mirror 26 of ~e laser cavi~.y. In odler words, ~e outside face 24b îs coated to transmit pumping light from the source S and to reflect light, that is produeed within the cavity, toward the output coupler 20. The quarte~wave plate 24 also causes the laser to produce single line multimode laser light that is characterized by two closely spaced apart orthogonal linear polarization of approximately ~e same power level. l'he quarterwave plate 24 defimes the polarization axes and introduces a hal~-wavelength pa*l difference between the polanzations. This shifts the orthogonal-mode beat frequency ~o half the cavity mode spacing, giving splittings of several GHz. In principle, polarized scatter or absorption in ~e quarter-wave plate 24 could degrade ~e polarization ratio; this can be offset by polarization-sensitive spatial hole burning e~fects that bias the system towards depolarized operation.
The aperture plate 22 is located between the lasant matenal 18 and ~e output coupler 20. Its pulpose is to avoid the production of high order transverse modes in the cavity.
If the source S comprises a plurality of laser diodes that are distributed along a re~erence axis (i.e., a diode array), the quarterwave plate 24 is located in the laser cavity to have its fast optical axis F aligned , ~ .
to the reference axis of the diode array.
In one particular embodiment, the lasant material was Nd:YAG and 2~ the source S was a semi-conductor or diode array that produced pumping light at about 1318 nm. Its output was 200rnW. Near threshold the laser operated in two closely-spaced, orthogonally-polarized modes (See Fig.
2). At higher powers, operation in several longitudinal modes was - ~ 8 . .
"` '; r WO 94/286~3 2 t ~1 ~ 9 9 PCT/US94/061Sl obser~ed. ~he polarization ratio remained within a few percent of unity at all tiTnes. The modal beat frequency was above 2 GH~.
'rhe output of the laser 10 is coupled to ~e optical modulator 16 by means of a noh-pol~rization maintaining single mode optical ~ber 12 and 5 a polarizer or polarization insensitive isolator 14. The polarizer produced plane polarized ligh~ (at about lS(~mW) for the caITier input to the modulator 16 (e.g, a Mach-Zehnder).
From ~e foregoing description, it will be observed that mlmerous vaIiations, alternatives and modi~lcations will be apparent to those skilled 10 in ~he art. Accordingly, this description is to be construed as illustrative only and is for ~e purpose of teaching those skilled in ~e art the Inanner of carrying out ~e invention. VaIious changes may be made, materials substituted and ~eatures of the invention may be utilized. For example, ~e source S can be focused by means of optics to pump ~e lasant material 15 18. It is also possible to in~roduce additional elements? such as etalons or birefringent filters, into ~e laser to restrict operation to only two modes.
This amolmts to extending the two-mode regime of operation, that has been obse~ved at lower power, to higher power ranges. The laser with the etalon still uses a quarter-wave plate to control the cavity mode 20 spacing and uses spatial hole burning to force two-mode operation. It is thus a simple modification of the apparatus just described. Thus, it will be appreciated that various modifications, alternatives, variations, etc., may be made without departing from the spirit and scope of the invention as defined in the appended claims. It is, of course, intended to cover by 25 the appended claims all such modifications involved within the scope of ~e claims.
.' .
Yet ano~er object of the invention is to provide a polarized op~ical 5 carrier for a communications system utili~ing an extemal optical modulator and a non-polarization maintaining optical fiber.
Still ano~er object of ~e invention is to proYide an apparatus and me~od for producing laser light cha~acterized by at least two longitudinal standing modes having two or~ogonal linear polarizations of about the 10 same power level.
One specific object of the ~nvention is to provide a Nd:YAG laser having a quarterwave plate to produce two polanzation eigenstates of substantially equal intensity and a single line ou~put.
In accordance with one embodiment of ~e presen~ invention, a 15 laser apparatus is provided comprising: a solid-state minimally bir~fringent lasant material having an input face and an opposite ou~ut face; an output coupler disposed towards said output face; pumping means for pumping light into said input face of said lasant material to pr~duce a population inversion therein; optical retardin~ means, having 20 one face abutting said input face of said lasant material and having an opposi~e face coated for transmitting light from said pumping means to said lasant material and for reflecting laser light produced by said rnaterial towards said output coupler to form an optical cavity for said - lasant material, for forming in said cavity single line multimode laser 25 light having at least two longitudinal standing laser modes characterized - by two closely spaced apart orthogonal linear polarizations of approximately the same power level; and means for avoiding high order transverse laser modes in said cavity.
: 3 .~
t WO ~4/28603 PCT/US94/06181 ~1~159~
In addition a method is disclosed for providing a polarized optical ca~Tier for use in a commlmications system of ~he type using an extemal modulator which operates in response to an optical calTier supplied through an optical fiber, comprising ~he steps of: forming an optical cavity ~or a lasant material having an input face and an opposite ou~ut face by iocating an oucput coupler disposed towards said output face and by locating a quarte~wave plate having one face abutting said input face and having an opposite face that is coated ~or transmitting optical pumping light to said lasant material and for reflecting laser light 0 produced by said lasant material towards said ou~Lpu~ coupler, plroviding a plurality of semi-conductors that, in response to the flow of electrical current therethrough, produces optical pumping light; purnping said lasant material using said semi-conductors to produce a population ir:version in - said lasant material; substantially reducing high order transverse modes in 15 said cavi~y; and adjusting said optical cavity to have an optical leng~ ~at is propo~ nal to one-half the longitudinal mode spacing of said laser light and to produce standing wave, substantially single frequency laser light that is characterized by two closely spaced apart orthogonal linear polarizations of about the same power level out of said output coupler for 20 at least some power levels of said semi-conductors.
Other advantages and feanlres of the present invention will become readily apparent from the following detailed description of the invention, the embodiments described therein, from the claims, and from the accompanying drawings.
2~
Brief De~ription of the.Drawings FIG. 1 is a schematic diagram of the laser that is the subject of the present invention; and :-~ 4 .
~ ~ 4~ ~ 99 ~;IG. 2 is depicts the perfo~nance of ~e laser illus~ated in FIG. 1.
While ~is invention is susceptible to embodiment in many different ~orms, there is shown in the drawings, and will herein be described in - detail, one speci~lc embodiment of ~e invencion. It should be understood, however, dlat ~e present disclosure is to be considered an e~emplification of ~e principles of the invention and is not intended to limit ~e invention to dle specific embodiment illustrated.
It is to be understood that the terms "light", "light pulse" and "laser energy" are used in this application not as limited to the visible spectrum of electromagnetic radiation~ but as in the broad sense of "radiant energy". :
Turning to FIG. 1, dlere is illustrated a diode-pumped solid-state laser 10, a non-polarization main~aining optical fiber 12, a polarizer 14 and an optical modulator 16. The laser 10 comprises a solid-state non-birefringent lasant material 18, an oucput coupler 20, an aperture plate 22, a quarterwave plate 24, and an input mirror 26. The lasant material 18 is optically pumped by a source S. `
Suitable optical pumiping means or sources S include, but are not limited to9 laser diodes, light-emitting diodes (including superluminescent diodes and superluminescent diode arrays~ and laser diode arrays, ` ` . ! i together with any ancillary packaging or structures. For the purposes hereof, the teIm "optical pumping means" includes any heat sink, 2~ thermoelectric cooler or packaging associated with said laser diodes, light-emitting diodes and laser diode arrays. For example, such devices are commonly attached to a heat resistant and conductive heat sink and a~e paclcaged in a metal housing. For efficient operation, ~e pumping means ~:~
; ~
S is desirably matched with a suitable absorption band of the lasant material 18. Although the invention is not to be so limited, a highly suitable optical pumping souree consists of a gallium aluminum arsenide laser diode, which emits light having a wavelength of about 810 nm, that is attached to heat sink. Heat sink can be passive in character. However, heat sink can also compromise a thermoelectric cooler or other tempera~re regulation means to help main~ain laser diode at a constant temperaturè and thereby ensure optimal operation of laser diode at a constant wavelength. It will be appreciated, of course, ~lat during operation the optical pumping means will be attached to a suitable power supply. Electrical leads from laser diode, which are directed to a suitable power supplyy are not illus~ated in dle drawings.
Conventional light-emitting diodes and laser diodes are available which, as a function of composition, produce output radiation having a 1~ waveleng~ over the range from about 630 nrn to about 1600 nm. Any device producing optical pumping radiation of a wavelength effective to pump a lasant material 18 can be used in the practice of this invention.
Por example, the waveleng~ of the output radiation from a GaInP based device can be varied from about 630 to about 700 nm by variation of the device composition. Similarly, the wavelength of the output radiation from a GaAlAs based device can be varied from about 750 to about 900 nm by variation of the device composition. InGaAsP based devices can be , used to provide radiation in the wavelength range from about lO00 to about 1600 nm.
If desired, the output facet of semiconductor light source S can be placed in butt-coupled relationship to input surface of the lasant material wi~out the use of inteImediate optics. As used herein, "butt-coupled" is defined to mean a coupling which is sufficiently close such that a "
.~
214~599 divergent beam of op~ieal pumping r3diation emanating from semiconductor light source or laser diode S will optically pump a mode volume within the lasant material 18 wi~ a sufficiently small transverse cross-xectional area so as to support essentially only single transverse 5 mode laser operation (i.e., TEMoo mode operation) in ~e lasant material.
Suitable lasant materials 18 include, but are not limited to, solids selected ~rom ~e group consis~ng of glassy and crystalline host materials which are ~oped with an active material and substances wherein ~e active material is a s~oichiometrie component of the lasant material. By way of 10 specific example, neodymium-doped YAG or Nd:YAG is a highly suitable lasant material for use in combination with an optical pllIIlpiIlg means S
which produces ligh~ having a waveleng~ of about 808 nun. When pumped with light of ~is waveleng~, neodymium-doped YAG can emit light having a wavelength of either about 1064 nm or about 1319 nm.
1~ Nd YAG is a non-birefringent lasant material. Preferably the lasant material îs "non-birefringent". By non-birefringent is meant ~at the gain medium has minimal birefringence in the direction of laser propagation.
Thus, a birefringent material like Nd:YLF can be used provided that ~e laser propagates along an optic axis of the crystal where it is "effectively"
20 nonbirefringent.
The lasant material 18 is located in an optical cavity which is fo~ ed by the output coupler 20 and the input mirror 26. l~e lasant method has two opposite ends 18a and 18b. One end 18a faces the output coupler 20 and the opposite end 18b faces the input mirror 26. When 2~ pumped by the source S, a population inversion is produced and the lasant material produces laser light.
The quarterwave plate 24 functions as a birefringent optical retarding means. It has one face 24a which abuts the input face 18b of dle WO 94/28603 PCT/VSg4/06181 ~141a99 lasant material 18, and it has an opposite or outside face 24b which receives pumping light from ~he source S. In one embodiment, the outside ~ace 24b is coa~d to function as the ~put mirror 26 of ~e laser cavi~.y. In odler words, ~e outside face 24b îs coated to transmit pumping light from the source S and to reflect light, that is produeed within the cavity, toward the output coupler 20. The quarte~wave plate 24 also causes the laser to produce single line multimode laser light that is characterized by two closely spaced apart orthogonal linear polarization of approximately ~e same power level. l'he quarterwave plate 24 defimes the polarization axes and introduces a hal~-wavelength pa*l difference between the polanzations. This shifts the orthogonal-mode beat frequency ~o half the cavity mode spacing, giving splittings of several GHz. In principle, polarized scatter or absorption in ~e quarter-wave plate 24 could degrade ~e polarization ratio; this can be offset by polarization-sensitive spatial hole burning e~fects that bias the system towards depolarized operation.
The aperture plate 22 is located between the lasant matenal 18 and ~e output coupler 20. Its pulpose is to avoid the production of high order transverse modes in the cavity.
If the source S comprises a plurality of laser diodes that are distributed along a re~erence axis (i.e., a diode array), the quarterwave plate 24 is located in the laser cavity to have its fast optical axis F aligned , ~ .
to the reference axis of the diode array.
In one particular embodiment, the lasant material was Nd:YAG and 2~ the source S was a semi-conductor or diode array that produced pumping light at about 1318 nm. Its output was 200rnW. Near threshold the laser operated in two closely-spaced, orthogonally-polarized modes (See Fig.
2). At higher powers, operation in several longitudinal modes was - ~ 8 . .
"` '; r WO 94/286~3 2 t ~1 ~ 9 9 PCT/US94/061Sl obser~ed. ~he polarization ratio remained within a few percent of unity at all tiTnes. The modal beat frequency was above 2 GH~.
'rhe output of the laser 10 is coupled to ~e optical modulator 16 by means of a noh-pol~rization maintaining single mode optical ~ber 12 and 5 a polarizer or polarization insensitive isolator 14. The polarizer produced plane polarized ligh~ (at about lS(~mW) for the caITier input to the modulator 16 (e.g, a Mach-Zehnder).
From ~e foregoing description, it will be observed that mlmerous vaIiations, alternatives and modi~lcations will be apparent to those skilled 10 in ~he art. Accordingly, this description is to be construed as illustrative only and is for ~e purpose of teaching those skilled in ~e art the Inanner of carrying out ~e invention. VaIious changes may be made, materials substituted and ~eatures of the invention may be utilized. For example, ~e source S can be focused by means of optics to pump ~e lasant material 15 18. It is also possible to in~roduce additional elements? such as etalons or birefringent filters, into ~e laser to restrict operation to only two modes.
This amolmts to extending the two-mode regime of operation, that has been obse~ved at lower power, to higher power ranges. The laser with the etalon still uses a quarter-wave plate to control the cavity mode 20 spacing and uses spatial hole burning to force two-mode operation. It is thus a simple modification of the apparatus just described. Thus, it will be appreciated that various modifications, alternatives, variations, etc., may be made without departing from the spirit and scope of the invention as defined in the appended claims. It is, of course, intended to cover by 25 the appended claims all such modifications involved within the scope of ~e claims.
.' .
Claims (20)
1. Laser apparatus, comprising:
a) a solid-state lasant material having an input face, an opposite output face, and a direction of propagation between said input face and said output face, said lasant material having minimal birefringence along said direction of propagation, b) an output coupler disposed towards said output face;
c) pumping means for pumping light into said input face of said lasant material to produce a population inversion therein;
d) optical retarding means, having one face abutting said input face of said lasant material and having an opposite face coated for transmitting light from said pumping means to said lasant material and for reflecting laser light produced by said material towards said output coupler to form an optical cavity for said lasant material, for forming in said cavity single line multimode laser light having at least two longitudinal standing laser modes characterized by two closely spaced apart orthogonal linear polarizations of approximately the same power level; and e) means for avoiding high order transverse laser modes in said cavity.
a) a solid-state lasant material having an input face, an opposite output face, and a direction of propagation between said input face and said output face, said lasant material having minimal birefringence along said direction of propagation, b) an output coupler disposed towards said output face;
c) pumping means for pumping light into said input face of said lasant material to produce a population inversion therein;
d) optical retarding means, having one face abutting said input face of said lasant material and having an opposite face coated for transmitting light from said pumping means to said lasant material and for reflecting laser light produced by said material towards said output coupler to form an optical cavity for said lasant material, for forming in said cavity single line multimode laser light having at least two longitudinal standing laser modes characterized by two closely spaced apart orthogonal linear polarizations of approximately the same power level; and e) means for avoiding high order transverse laser modes in said cavity.
2. The apparatus of claim 1, further including:
f) a non-polarization maintaining optical fiber having an input end connected to receive laser lightwave from said output coupler and having an opposite output end.
f) a non-polarization maintaining optical fiber having an input end connected to receive laser lightwave from said output coupler and having an opposite output end.
3. The apparatus of claim 2, further including:
g) a polarizer for receiving light from said output end of said optical fiber and for producing a plane polarized light output.
g) a polarizer for receiving light from said output end of said optical fiber and for producing a plane polarized light output.
4. The apparatus of claim 3, further including:
h) an optical modulator for receiving light passing through said polarizer from said output end of said optical fiber.
h) an optical modulator for receiving light passing through said polarizer from said output end of said optical fiber.
5. The apparatus of claim 1, wherein said lasant material is non-birefringent.
6. The apparatus of claim 1, wherein said pumping means comprises an array of laser diodes that are distributed along a reference axis; and wherein said optical retarding means comprises a quarterwave plate having a fast optical axis aligned to said reference axis of said laser diode array and having a slow optical axis at right angles to said fast optical axis.
7. The apparatus of claim 1, wherein said cavity has an optical length that is proportional to one-half the mode spacing of said laser light.
8. The apparatus of claim 1, wherein said optical retarding means comprises a quarterwave plate whose fast and slow optical axes are in a plane parallel to said input face of said lasant material.
9. The apparatus of claim 8, wherein said optical retarding means includes at least one of an etalon and birefringent filter means.
10. The apparatus of claim 1, wherein said means for avoiding high order transverse laser modes in said cavity comprises an aperture plate located between said output coupler and said input face of said lasant material.
11. A multi-mode laser, comprising:
a) a mass of lasant material having minimal birefringence between an input face and an opposite output face;
b) an output coupler disposed towards said output face;
c) at least one semi-conductor that, in response to the flow of electrical therethrough, pumps laser light into said input face of said lasant material for producing a population inversion in said lasant material; and d) input means for transmitting light from said semi-conductor to said lasant material and for reflecting laser light produced by said mass towards said output coupler to form an optical cavity for said lasant material, said optical cavity having optical length that is proportional to one-half the longitudinal mode spacing of said laser light; and e) birefringent means for optically retarding one of two polarization eigenstates relative to light in said cavity of the other polarization eigenstate by an amount to substantially eliminate phase coherence between said eigenstates, and for producing in said cavity two polarization eigenstates of substantially equal intensity and substantially single line multi-mode laser light.
a) a mass of lasant material having minimal birefringence between an input face and an opposite output face;
b) an output coupler disposed towards said output face;
c) at least one semi-conductor that, in response to the flow of electrical therethrough, pumps laser light into said input face of said lasant material for producing a population inversion in said lasant material; and d) input means for transmitting light from said semi-conductor to said lasant material and for reflecting laser light produced by said mass towards said output coupler to form an optical cavity for said lasant material, said optical cavity having optical length that is proportional to one-half the longitudinal mode spacing of said laser light; and e) birefringent means for optically retarding one of two polarization eigenstates relative to light in said cavity of the other polarization eigenstate by an amount to substantially eliminate phase coherence between said eigenstates, and for producing in said cavity two polarization eigenstates of substantially equal intensity and substantially single line multi-mode laser light.
12. The laser of claim 11, wherein said lasant material is Nd:YAG; and wherein said longitudinal mode spacing is on the order of 2 GHz.
13. The laser of claim 11, wherein said output coupler is coated to preferably reflect light at 1319 nm relative to light at 1338 nm.
14. The laser of claim 11, wherein said input means comprises a quarterwave plate having one face abutting said input face of said lasant material and having an opposite face abutting said input face.
15. The apparatus of claim 11, further including an aperture plate located between said output coupler and said input face of said lasant material.
16. In a communications system of the type using an external modulator which operates in response to an optical carrier supplied through an optical fiber, a method of providing a polarized optical carrier comprising the steps of:
a) forming an optical cavity for a lasant material having an input face and an opposite output face by locating an output coupler disposed towards said output face and by locating a quarterwave plate having one face abutting said input face and having an opposite face that is coated for transmitting optical pumping light to said lasant material and for reflecting laser light produced by said lasant material towards said output coupler, b) providing a plurality of semi-conductors that, in response to the flow of electrical current therethrough, produces optical pumping light;
c) pumping said lasant material using said semi-conductors to produce a population inversion in said lasant material;
d) substantially reducing high order transverse modes in said cavity; and e) adjusting said optical cavity to have an optical length that is proportional to one-half the longitudinal mode spacing of said laser light and to produce standing wave substantially single frequency laser light characterized by two closely spaced apart orthogonal linear polarizations of about the same power level out of said output coupler for at least the some power levels of said semi-conductors.
a) forming an optical cavity for a lasant material having an input face and an opposite output face by locating an output coupler disposed towards said output face and by locating a quarterwave plate having one face abutting said input face and having an opposite face that is coated for transmitting optical pumping light to said lasant material and for reflecting laser light produced by said lasant material towards said output coupler, b) providing a plurality of semi-conductors that, in response to the flow of electrical current therethrough, produces optical pumping light;
c) pumping said lasant material using said semi-conductors to produce a population inversion in said lasant material;
d) substantially reducing high order transverse modes in said cavity; and e) adjusting said optical cavity to have an optical length that is proportional to one-half the longitudinal mode spacing of said laser light and to produce standing wave substantially single frequency laser light characterized by two closely spaced apart orthogonal linear polarizations of about the same power level out of said output coupler for at least the some power levels of said semi-conductors.
17. The method of claim 16, wherein step (b) is performed by using an array of said semi-conductors distributed along a reference axis for end pumping said lasant material; and wherein step (a) is performed by using a quarterwave plate having an axis that is generally parallel to said reference axis of said array.
18. The method of claim 17, wherein step (d) is performed by locating in said optical cavity an aperture plate between said output coupler and said output face of said lasant material.
19. The method of claim 16, further including the steps of:
f) optically connecting one end of a non-polarization maintaining optical fiber to said output coupler; and g) connecting a polarizer between said modulator and the other end of said optical fiber.
f) optically connecting one end of a non-polarization maintaining optical fiber to said output coupler; and g) connecting a polarizer between said modulator and the other end of said optical fiber.
20. The method of claim 16, wherein step (a) is performed by using a lasant material formed from a crystal of Nd:YAG; wherein step (b) is performed by using semi-conductors that produce light at about 800 nm; and wherein step (c) is performed to produce laser light having a wavelength of substantially 1319 nm.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US7150493A | 1993-06-02 | 1993-06-02 | |
| US08/071,504 | 1993-06-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2141599A1 true CA2141599A1 (en) | 1994-12-08 |
Family
ID=22101743
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2141599 Abandoned CA2141599A1 (en) | 1993-06-02 | 1994-06-02 | Depolarized solid-state diode-pumped laser |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0657070A1 (en) |
| JP (1) | JPH08500213A (en) |
| CA (1) | CA2141599A1 (en) |
| WO (1) | WO1994028603A1 (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7286241B2 (en) | 1999-06-24 | 2007-10-23 | Lockheed Martin Corporation | System and method for high-speed laser detection of ultrasound |
| US6483859B1 (en) | 1999-06-24 | 2002-11-19 | Lockheed Martin Corporation | System and method for high-speed laser detection of ultrasound |
| CN116565678B (en) * | 2023-07-10 | 2023-10-03 | 北京凯普林光电科技股份有限公司 | Laser device |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4725787A (en) * | 1986-12-31 | 1988-02-16 | The United States Of America As Represented By The Secretary Of The Army | Phase-conjugated hybrid slab laser |
| US5031182A (en) * | 1989-05-18 | 1991-07-09 | Amoco Corporation | Single-frequency laser of improved amplitude stability |
| FR2658367B1 (en) * | 1990-02-13 | 1992-06-05 | Sagem | LASER PROVIDING TWO WAVES AT DIFFERENT FREQUENCIES. |
| JP2893862B2 (en) * | 1990-05-16 | 1999-05-24 | ソニー株式会社 | Solid state laser oscillator |
-
1994
- 1994-06-02 JP JP7501060A patent/JPH08500213A/en active Pending
- 1994-06-02 WO PCT/US1994/006181 patent/WO1994028603A1/en not_active Application Discontinuation
- 1994-06-02 EP EP94919319A patent/EP0657070A1/en not_active Withdrawn
- 1994-06-02 CA CA 2141599 patent/CA2141599A1/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| EP0657070A1 (en) | 1995-06-14 |
| EP0657070A4 (en) | 1995-04-12 |
| JPH08500213A (en) | 1996-01-09 |
| WO1994028603A1 (en) | 1994-12-08 |
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