Tunable lasers handbook phần 5 pptx

The use of cw dye laser oscillators as injection sources of amplification stages utilizing ring cavity configurations is discussed by Blit et al.. hAr+ laser power from [3] and Kr+ laser

(Reprinted with permission from Duarte et a/ [72] and Elsevier Science.)

Schematics of (a) flashlamp-pumped MPL oscillator and (b) HMPGI oscillator

In this regard, the narrow-linewidth emission pulse must be synchronized to arrive during the buildup period of the forced-oscillator pulse In the case of forced oscillators using unstable resonator optics, the magnification of the optics must be optimized relative to the beam dimensions of the master oscillator to completely fill the active volume of the forced oscillator Also the injection beam should be aligned exactly for concentric propagation along the optical axis of the forced oscillator The performance of flashlamp-pumped master-oscillator/ forced-oscillator systems is listed in Table 10 In addition to those results, energy gains as high as 478 have been reported for an MPL master oscillator and a forced oscillator with a magnification factor of 5 [62]

The use of cw dye laser oscillators as injection sources of amplification

stages utilizing ring cavity configurations is discussed by Blit et al [78] and Tre-

hin et a / [79]

4 cw LASER-PUMPED DYE LASERS

The cw dye lasers span the spectrum from -370 to -1000 nm Frequency doubling extends their emission range into the 260- to 390-nm region An impor- tant feature of cw dye lasers has been their ability to yield extremely stable emissions and very narrow linewidths These qualities have made cw dye lasers

5 Dye Lasers 185

FIGURE 9

sion from Duarte et al [72] and Elsevier Science.)

Partial view of ruggedized multiple-prism grating oscillator (Reprinted with permis-

extremely important to applications in physics, spectroscopy, and other sciences

A thorough and extensive description of this branch of dye lasers is given by Hollberg [3] Here some of the most important features of cw dye lasers and their emission characteristics are surveyed

4.1 Excitation Sources for cw Dye Lasers

The main sources of excitation for cw dye lasers are the argon ion (AI-+) and the krypton ion (Kr+) lasers These are conventional discharge lasers that emit

186 F.J Duarte

TABLE 8

Master Oscillatora

Optimum Performance of Ruggedized Multiple-Prism Grating

Output energy (mJ) A\’ (hIHz) 6%’h AB (mrad) C (rnkl)

aFrom Duarte er al [72], with permission

TABLE 9 Performance of Ruggedized Multiple-Prism Grating Master

Oscillator Prior (First Row) and Following (Second Row) Field Testa

Output energy (mJ) Av (MHz) 6k% AB (mrad) C (mhl)

blaster oscillator configuration energy Energy gain Reference

T s o etalons Flat-mirror cavity 600 mJ at 589 nm 200 [751

Trio etalons Flat-mirror cavity 300 mJ at 590 nm [771

OXdapted from Duarte [37] with permission

via excitation mechanisms such as Penning ionization [go] Table 11 lists some

of the most widely used transitions in dye laser excitation Note that the quoted powers are representative of devices available commercially It should also be indicated that not all transitions may be available simultaneously and that more than one set of mirrors may be required to achieve lasing in different regions of the spectrum Also, for a mirror set covering a given spectral region, lasing of individual lines may be accomplished using intracavity prism tuners

5 Dye Lasers 187 TABLE 1 1 Excitation Lasers of cw Dye Lasers

Laser Transitiona Wavelength (nm) Powerb (W)

5p4p; - 5s2P,,,

5p4P9: - 5s4Px,i 5p4PQ1 - 5S4P,,>

.5p4Do, - 5S2P,,?

528.69 514.53 501.72 496.51 487.99 476.49 472.69 465.79 457.93 4.54.50 799.32 752.55 676.4 647.09 568.19 530.87 520.83

1.5 10.0

1 .s 2.5 7.0 2.8 1.2 0.75 1.4

1 .o 0.1 0.35 0.2 1.4 0.53 0.7 0.25

~~~~~~~~ ~~~~ ~ ~ ~

OTransition identification from [SO]

hAr+ laser power from [3] and Kr+ laser powers from [81]

Given the relatively long cavity length of these lasers (typically -1 m), and their narrow beamwaists (-1 mm), the output beam characteristics are excellent

In this regard these 1a.sers can offer single-transverse-mode outputs and beam divergence’s approaching the diffraction limit

In addition to the output powers listed in Table 11, higher powers are avail-

able For instance, Baving et al [82] reports the use of a 200-W multiwave- length A@ laser in the excitation of a linear cw dye laser The Ar+ laser oscillated simultaneously at 457.93, 476.49,487.99,496.51, 501.72, and 514.53 nm Other lasers useful in the excitation of cw dye lasers include HeNe [83,84], frequency- doubled cw Nd:YAG [3], and semiconductor lasers

4.2 cw Dye Laser Cavities

The cw dye laser cavities evolved from the simple and compact linear cavity first demonstrated by Peterson et al [85] External mirrors and intracavity tuning prisms were introduced by Hercher and Pike [86] and Tuccio and Strome [25]

(Fig 11) An important innovation in cw dye lasers was the introduction of the dye jet [83] Fast flow of the dye solution at speeds of a few m-s-1 is important

188 F J Duarte

Stainless

Brewster Angle Prism Argon Laser Beam

514.5 nm

Dye Laser Beam FIGURE 1 1

and Strome [XI.)

Linear cw dye laser cavity configuration (Reprinted 41th permission from Tuccio

to induce heat dissipation and hence reduce thermally induced optical inhomo- geneities in the active medium [85]

Widely used configurations of cur dye laser cavities include the three-mirror folded linear cavity (see, for example, [20] and references therein) and ring-dye laser cavities (see, for example, [3] and references therein) These two configura- tions are shown in Fig 12 In both cases excitation from a cw laser is accom- plished semilongitudinally to the optical axis defined by M, and M, Tuning ele- ments or frequency-selective elements (FSEs), are deployed between h/I, and

M, in the linear cavity, and between M, and M, in the ring cavity The unidirec- tional device (UDD) depicted in Fig 12(b) is an optical diode that controls the direction of propagation in the ring cavity [3]

Ring-dye laser cavities circumvent the problem of spatial hole burning associ- ated with linear cavities [3] Also ring cavities are reported to yield higher single- longitudinal-mode power than linear cavities [3] However, linear configurations offer greater optical simplicity and lower oscillation thresholds

Diels [87] discusses the use of propagation matrices, applicable for Gauss- ian beam propagation analysis, to characterize stability conditions and astigma- tism in cw dye laser cavities

Linewidth narrowing and FSEs used in cw dye lasers are birefringent crys- tals prisms, gratings, and Fabry-Perot etalons Often two or more FSEs are nec- essary to achieve single-longitudinal-mode oscillation The first stage in the fre- quency narrowing usually consists of utilizing prisms or birefringement filters to yield a bandwidth compatible with the free spectral range (FSR) of the first of two etalons In turn, the second etalon has a FSR and finesse necessary to restrict oscillation in the cavity to a single-longitudinal mode [3] Alternative approaches may replace the second etalon by an interferometer [88] The performance of var- ious linear and ring cw dye lasers is listed in Table 12

5 Dye Lasers 189

Dispersive and/or FSE Dye Jet

C

a

Dispersive and/or

Dye Jet FIGURE 1 2

(see text for details) (Reprinted with permission from Hollberg [3].)

(a) Three mirror-folded linear cw dye laser cavity (b) A cw ring dye laser cavity

about 3 ps Hence, frequency stabilization techniques should offer rapid response

Hollberg [3] lists and describes in detail a number of frequency stabilization techniques:

Cavity side lock 13,931: A beamsplitter directs a fraction of the laser out-

put toward a second beamsplitter that distributes the signal toward a detec- tor and a reference Fabry-Perot interferometer The difference between the

190 F.J Duarte

TABLE 12 Performance of cw Dye Laserso

Cavity coverage (nm) 03') Linewidth Efficiency Reference

Ring 364-524 using 4 d>es 0.43 SLMd 10.1 ~911

Using rhodamine 6G

Using coumarin 102

oUnder Ar+ laser excitation

hbfaximum cw power quoted was 52 W for a pump power of 175 W

COutput power without intracavity tuning prism is quoted at 43 W for a pump power of 200 W

"ingle-longitudinal mode ISLMI Linewidth values can be in the few megahertz range

direct signal and the signal from the reference cavity is used to drive the laser cavity servocontrol amplifier

Modulation lock [3]: A beamsplitter sends part of the emission beam toward a reference Fabry-Perot interferometer The transmitted signal from the reference cavity is compared at a lock-in amplifier with the sig- nal modulating the dye laser frequency The resulting error signal is used

to drive the dye laser cavity servo control

t -optical hetel-od~ne lock [3,94]: A beamsplitter sends portion of the dye laser output toward a phase modulator (electro-optics transducer) The phase-modulated radiation then propagates toward a reference cavity via a Thompson prism in series with a Faraday rotator The return beam from the reference cavity is reflected by the Thompson prism toward a detector The signal from the detector is sent to a set of filters followed by

a balanced mixer At this stage the signal from the reference cavity is mixed with the signal from the phase modulator to produce an error sig- nal that drives the dye laser cavity servocontrol

Post-laser stabi1i:ation [3,92]: This method changes the frequency of the dye laser emission outside the cavity The technique combines an electro-optic modulator (EOM) and an acousto-optic modulator (AOM)

to yield a fast frequency transducer The EOM and the ,40M are deployed in series with the EOM in between two mirrors whose optical axis is at a slight angle relative to the propagation axis of the laser beam The aim of the mirrors is to provide an optical delay line (the beam

Further frequency stabilization methods use molecular media, such as iodine to provide frequency reference [95] Performance of frequency-stabilized

cw dye lasers is tabulated in Table 13

The dye laser with its continuous and wide frequency gain profile is an inherent source of ultrashort temporal pulses Indeed the development of femtosecond-pulsed dye lasers has been essential to the development and advancement of ultrashort-pulse laser science An excellent review on this sub- ject including a historical perspective is given by Diels 1871 In this section the performance of femtosecond-pulsed dye lasers is presented together with a description of technical elements relevant to the technology of ultrashort-pulse laser emission

For a comprehensive discussion on ultrashort-pulse-measuring techniques the reader should refer to Diels [87] Also, for alternative methods of ultrashort- pulse generation utilizing distributed feedback dye laser configurations, the review given by Schafer [98] is suggested

The principles and theory of femtosecond-pulse generation has been dis- cussed by many authors [99-1091 Notable among these works are the papers by Zhakarol and Shabat [99] Diels et 01 [loo], and Salin et a! [loll, which discuss nonlinear effects and the subject of solitons Pulse evolution is discussed by New [102] An important contribution of general interest is that of Penzkofer and Baumler [10131 This comprehensive work includes excitation parameters and cross sections relevant EO the saturable absorber DODCI and the gain dye rhodamine 6G

5.1 Femtosecond-Pulse Dye Laser Cavities

Mode locking in dye lasers using an intracavity saturable absorber dye cell was first demonstrated in a flashlamp-pumped dye laser [ 1101 This development was followed by the demonstration of passive mode locking in a linear cw dye laser [ 11 I]

A development of crucial importance to the generation of ulti-ashort pulses was the introduction of the concept of colliding-pulse mode locking (CPM) by

192 F.J Duarte

TABLE 13 Performance of Frequency-Stabilized cw Dye Lasers

Frequency Limiting Stabilization method Linewidth drift factors Reference

Cay18 side lad Uses I50 kHza (rms) 50 hlHz/hou [961 two Fabry-Perot interferometers

rf-optical hereradyie lock 100 Hz Sen70 electronics [91] Uses signals reflected from

a reference cavity <750 Hzn 720 Hz/sec Mechanical noise [97]

and electro-optic modulatorsh

UEmission source: ring-dye laser

!'For dye lasers with inmnsic linewidths of -1 MHz this method has produced linewidths of -1 lcHz [3]

Ruddock and Bradley [112] Subsequently, Fork et al [113] incorporated the CPM concept to ring cavities, thus demonstrating pulses as short as 90 fs CPM is established when a colIision between two counterpropagating pulses

is induced at the saturable absorber The interaction of the tu o counterpropagating pulses gives origin to interference that induces a reduction in the pulse duration Two of the most widely used cavities in femtosecond dye lasers are the cw linear and ring cavity configurations modified to incorporate CPM Linear and ring femtosecond dye laser cavities incorporating the saturable absorber region in its counterpropagating arrangement is shown in Fig 13 In both cavities the gain region is configured in the optical axis defined by M, and M,, whereas the sat- urable absorber is deployed in the optical path defined by M, i d M4 Note that in both instances these ultrashort-pulse cavities are equivalent to the linear and ring

cw dye laser cavities shown in Fig 12 with M, replaced by the CPM arrangement

An additional feature of these cavities is the use of intracavity prism to induce pulse compression In dye lasers, pulse broadening by positive group veloc- ity dispersion (GVD) is induced at the dye gain and absorber regions Multiple- prism arrangements can be configured to provide net positive dispersion, no dis- persion, or negative dispersion [ 1,1071 In femtosecond dye lasers, intracavity prisms are deployed to provide negative GVD and hence compensate for the posi- tive GVD generated at the dye regions

Gordon and Fork [ 1041 provide an expression for the group velocity disper- sion constant in a prism array:

D=[&)g,

where L is the physical length of the light path, and P is the optical path length

through the prism array By differentiating

5 Dye Lasers 193

R=5cm lntracavity Quartz Prisms

Fork et al [ 1051 have shown that

which shows the dependence of d’P/dhl on d@/dn and d2@/dii2 It is these two

derivatives, d@ldn and d ~ @ / d n ~ , that depend on the overall prismatic dispersion

A negative value for d T / d h z can be achieved by adjusting the interprism separa- lion The effect of minute geometrical perturbations and/or beam deviations on overall dispersion was quantified by [107] This work demonstrated that very small angular deviations induced changes in dispersion that can only be assessed using the generalized multiple-prism dispersion theory Generalized expressions

for d@/dn and &@/dnL x e given in Chapter 2

194 F J Duarte

Kafka and Baer [lo81 and Duarte [lo71 have discussed the effect of varia- tions of beam angle of exit and incidence on overall dispersion Bor [I091 has considered the distortion of femtosecond pulses following transmission in lens sys tems

The first Qse of intracavity prismatic dispersion to achieve pulse compres- sion was reported by Dietel et al [ 1151 These authors reported pulse lengths of less than 60 fs A collinear four-prism sequence was introduced by Fork et al

[lOS] and a single prism pair was used by Diels et al [ 1001 The dispersion the- ory of multiple-prism arrays has been discussed by Duarte [1,106.107] Table

14 lists the performance of several prismatic configurations Table 15 tabulates relevant values of dn/dh and d*n/dhz for several prism materials Note that

some materials such as LaSF9 and ZnSe provide significantly higher drildh and

8 n / d h ? values that enable the design of very compact multiple-prism pulse compressors [SO]

An alternative and/or complementary avenue to prismatic pulse compression

is the use of grating pairs In this regard, Fork et al [6] report the use of an extra- cavity four-prism compressor in conjunction with two grating pairs to achieve pulses as short as 6 fs These authors note that the shortest pulse measured using the grating pairs alone, in the external compressor, was 8 fs An additional fea- ture of this work was the preamplification of 50-fs pulses, generated in a cavity including CPM and prismatic GVD compensation, to energy levels of - 1 mJ and

a prf of 8 kHz The amplification pump source was a CVL laser [6]

Amplification of 70-fs pulses to gigawatt power levels has been reported by Fork et al [118] These authors employed an extracavity grating pair following the multiple-amplification stages

Diels [87] has tabulated a comprehensive performance listing of ultra-fast dye lasers utilizing passive and hybrid mode locking This listing is reproduced

aFirst report on the use of prismatic inuacavity dispersion to achieve GVD compensation (1983)

bFirst report on the use of a compensating prism pair to achieve GVD compensation (1985)

<First report on the use of two compensating prism pairs to achieve GVD compensation (1984)

5 Dye Lasers 195

6 SOLID-ST.ATE DYE LASERS

Solid-state dye lasers were first demonstrated by Soffer and McFarland

[ 1391 in 1967 using rhodamine-doped polymethyl methacrylate (PMMA4) under laser excitation Lasing of rhodamine-doped PMMA under flashlamp excitation was demonstrated by Peterson and Snavely [ 1401

Table 18 lists available matrices used in solid-state dye lasers Modified PMMA (MPMMA) [141,142] is an improved form of PMMA with high damage thresholds and excellent optical properties MPMMA results from purifying the initial monomer compositions and by doping PMMA with low molecular addi-

tives [142] Gromov et ul [141] report that MPMMA has an energy damage

thresholds of 13 J/cm’ Further, these authors report that the threshold for photo- bleaching of rhodamine 6G in MPMMA is -1.6 J/cmz Duarte [46] reports that for a beam radius of 200 pm no e\ridence of photobleaching in rhodamine-doped MPMMA was evident at energy densities of -0.7 J/cmz The measured refractive index for rhodamine-6G-doped MPMMA at a concentration of 0.1 mM is 1.453

For QRMQSIL, Duarte et al [45] report on long-pulse lasing under dye

laser excitation This QRMOSIL was synthesized using the method of Dunn er

al [114] and was composed by a 1:1:1:3.5 molar ratio of TMQS/MMA/3- (trirnethoxysilyl) propyl MA/0.03 N HC1 [145] The dye concentration used in

TABLE 15 Dispersion Characteristics of Prism Materials for Pulse Compression0

2.511

0.62 0.62 0.62 0.62 0.62 0.80 0.62

0.80

-0.03059 -0.0361 3 -0.07357 -0.10873 -0.1 1189 -0.05201 -0.698 -0.246

0.1267 [lo51 0.15509

0.31332 [871 0.53819 [871 0.57778

0.18023 5.068 1.163

“Adapted from Diels [87], with permission

hcalculated using data from Marple [117]

TABLE 16 Passive Mode Locking

493-502 488-5 12 494-5 12 492-5 12 492-507 553-570 553-570 570-600

6 16-658 652-68 1

652494

655673 727-740 762-778 742-754

Pump Ring cavity, energy transfer

“Adapted from Diels [87], with pennission

‘>See Appendix for abbreviations

TABLE 17 Hybrid Mode Locking0

Oxazine 720

DQTCI DDI DOTCI DDI, DOTCI HITCI HlTCl HlTCl

IR 140 DaQTeC

Direct pumping with doubled

Nd:YAG, linear cavity Linear cavity Linear cavity Linear cavity Linear cavity Linear cavity Linear cavity Linear cavity Ring cavity Linear cavity t7Adapted from Diels 1871, with pennission

d &e Appendix for nbl>reviations

9

Modified polymethyl methacrylare MPMMA

Tetraethoxysilane [Si(OCIH,),] TEOS

Tetramethoxysilane [Si(OCH3),] TMOS

Organically modified silicate ORMOSIL

Silica-PMMA nanocotnposites

N e t h y l methacrylate (MMA) is CH,= C(CH,)COOCH,

these experiments was 2 mM and the excitation laser was a flashlamp-pumped dye laser with a 170-11s pulse duration [45]

An alternative to ORMOSILs are the transparent silica-PMMA nanocom- posites [ 1461 Although these nanocomposites have a number of similarities with ORMOSILs, including very high transparencies, they also have a number of syn- thetic differences [ 1461 These nanocomposites can be optically polished to yield high-quality optical surfaces This feature coupled to their high transparency offers a very attractive optical material Silica-PMMA nanocomposites doped with rhodamine 6G and rhodamine B have been made to lase under transverse excitation from a coumarin 152 dye laser [147]

An important difference between PMMA-type matrices and silicate matrices

is the internal structure of the latter This structure can induce refractive index variations that leads to optical inhomogeneities of the active medium These inhomogeneities can be characterized by propagating a narrow-linewidth laser through the matrix and then observing the far-field interferometric pattern thus produced [ 1471

The energetic and efficiency performances of solid-state dye lasers using a variety of host matrices are listed in Table 19

The performance of solid-state dispersive dye laser oscillators is given in Chapter 2 for MPMMA matrices In addition to those results, Duarte et al [45] reports an output energy of - 1 m J at Av = 3 GHz for a multiple-prism grating oscillator using TEOS doped with rhodamine 6G at 2 mM The same oscillator yielded <1 mJ/pulse for ORMOSIL doped with rhodamine 6G at the same con- centration In an extension of the work published in 1461 Duarte [I531 has opti- mized the architecture of the solid-state multiple-prism grating dye laser oscilla- tors and has demonstrated a very compact dispersive oscillator This dispersive oscillator has a cavity length in the 55-60 mm range and yields efficient single- longitudinal-mode lasing at AV = 320 MHz with a near-Gaussian temporal pro- file 3 4 ns in duration (FWHM)

TABLE 1 9 Performance of Solid-state Dye Lasers

Rhodamine 6G

11B PM-570h Sulforhodamine 640

Rhodamine 6G

Rhodamine 6G Rhodamine 6G

Rhodamine 6G

flashlamp flashlamp

Nd:glass lasera

Nd:YAG laserf!

Nd:YAG laser"

Nd lasefl Flashlamp-pumped dye l a s e r Nd:YAG lase^

(The dye used was coumarin 525

dThe pulse length was 60 ns (FWHM)

APPENDIX OF LASER DYES

weight absorption fluorescence lansing 1 (nrn) rangeU

417

(Nd

41 9 (N2)

APPENDIX OF LASER DYES (confinued)

Name

Molecular Maximum M a x i m u m M a x i m u m Tuning

weight absorption fluorescence laming I (nrn) ronge"

N APPENDIX OF LASER DYES (continued)

N APPENDIX OF LASER DYES (continued)

Name

weight absorption fluorescence lonsing I (nm) rangeo

td APPENDIX OF LASER DYES (continued)

weight absorption fluorescence lansing I (nm) rangea

Name I 4 b m l I(nm1 (pump laser) (nm) Solvents Molecular Structure

568 (Nd:YAG)C