CA1178702A - Cascaded modelocked laser system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A cascaded laser system providing optimum stable shot-to shot reproducible modelocking of high power lasers, particularly gain or Q switched lasers, such as high pressure C02TEA lasers and Nd:glass lasers. The system combines both active and passive modelocking techniques so that the stable generation of ultra short, high power pulses of fixed timing is provided over a wide range of system parameters. In the system, active modelocked pulses generated by a relatively low power laser modulates the loss of a saturable absorber associated with a relatively high power laser in such a manner that during buildup of modelocking the high power laser exhibits the characteristics of an actively modelocked laser but thereafter exhibits the characteristics of a passively modelocked laser.

Description

i~7~702 BACKGROUND OF THE INVENTION
. . _ . . _ The present invention relates to laser systems and more particularly to modelocked laser systems. A modelocked laser is well known, as defined on page 957 of the Dictionary of Scientific and Technical Terms, Daniel N. Lapedes, ed., McGraw-Hill (New York, 1976) as: "A laser designed so that several modes of oscillation with closely spaced wavelengths, in which the laser would normally oscillate, are synchronized so that a pulse of light, lasting for as little as a picosecond, is generated.".
Modelocked laser systems incorporating modulators and saturable absorbers are well known and are classified as ei'her active or passive systems. In such systems sinusoidal intracavity modulation at the cavity mode spacing or a multiple thereof provides coupling among the axial modes of the laser (active modelocking) or a saturable absorber acts as a fast gate within the cavity which opens and shuts with the passage of each modelocked pulse (passive modelocking).

li~7OZ

These systems lnvolve four important considerations, namely stability, power handling capability, timing and output pulsewidth. The advantages and disadvantages between active and passlve modelocking techniques can be summarized a~ follows.
Regarding stabillty, an active system provldes excellent stability since the intracavity modulation provides coupling among the modes from the onset of emission (onset regime) until steady state lasing (steady state regime) is achieved. Equivalently, the opening and closing of the modulator selects that portion o~
the spontaneous emission in a round trip transit time which is to be amplified and thus modelocking repeats reliably from ~hot to shot in gain or Q switched lasers. Wlth respect to passive systems, on the other hand, stabillty is relatively poor since the build-up of a laser pulse from noise relies on the selection of the highest noise spike in a cavity transit time due to the preferential saturation of the absorber Thus a delicate balance of gain and loss in the cavity is required to reach steady state modelocked operation This is particularly damaglng in galn or Q switch lasers where the gain fluctuates significantly from pulse to pulse and where, in fact, satellite pulses, pulse-width ~luctuations,and other indications of incomplete mode-locking are commonly observed As to power, in active systems the intenslties possible in the cavity are limited by the power handling capability of the modulator With respect to a passive system, on the other hand, the existence of gaseou~ dye absorbers allowsfor high power ~ 5te ~
operation and~is limited only by the power handling capability of the laser medium or other cavity elements such a~ windows or mirrors Nevertheless, serious limitat~ons still exist where 3o modelocking of relatively high power lasers is desired 1~7870Z

Wlth respect to timing, pulses appear ln a flxed sequence relatlve to the modulator in an actlve system whereas in a passive ~ystem the pulsed timing fluctuates from shot to shot since there i9 no timing reference, i.e. pulse can occur anywhere in the transit time Finally, as to pulsewidth, active systems do not produce the shortest possible pulses because sinusoidal modulation provides coupling only among ad~acent axial modes whereas in passive systems shorter pulses are generated in actlve mode locking because the modulation of the absorber lnduced by the passage of the pulse itself is faster than the sinusoidal and hence rich in higher harmonics which couple over a broader range of the mode t S spectrum.
Thus to ensure stable repetitive operatlon and a fixed timing of a modelocked train, it would be advantageous to select an active modelocking scheme whereas to generate ultra short pulses which utilize the full bandwidth and available power of the laser passlve mode locking would be preferable.
In the system of the present invention to be sub-sequently described, both active and pas~ive modelocking tech-nique8 are combined in ~uch a manner that the stable generation A of ultr~ short, hlgh powered pulses of fixed timing resultSover a wide range of system parameters Accordingly, lt is an ob~ect of the present invention to provide an improvement in modelocked lasers.
It is another ob~ect of the pre~ent invention to provide a modelocked laser system which incorporates the ad-vantages of both active and passive modelocking while eliminating the respective drawbacks.
Still another ob~ect of the present invention is to provide a modelocked laser system which has an improved stabllity and power handling capability.

i1 117870;:

Stlll a ~urther object of the present invention ls ¦to provide a modelocked laser system whlch exhiblts ~mproved . timing and pulsewidth characterlstlcs, Summary These and other ob~ects of the present invention are accomplished by means of a cascaded modelocked laser system lncluding a pair of modelocked lasers wherein the train Or actlvely modelocked pulses generated by a first laser modulates th~ , loss of a saturable absorber located on the beam axis of a Becond 0 laser and wherein the cavities of the two laser systems are rranged in a first embodiment in a tandem, or series, configura-ion while in a second embodiment are arranged in a crossed, or intercepting, cavity configuration. During an onset regime during which laser pulse build-up occurs in the second laser, the system exhibits the characteristics of an actively modelocked laser but thereafter, during a steady state regime, exhibits the jcharacteristics of a passively modelocked laser.

¦Brief Description of the Drawin~s ¦ Fi~ure 1 ls a schematic representatlon of a ~irst ¦embodiment of the sub~ect invention;

¦ Fi~ure 2 is a schematic representation of a second ¦embodlment of the sub~ect invention;

l Figure 3 i8 a set of graphs illustrative Or the opera-¦tion Or a conventional passive modelocked laser; and ¦ Figure 4 is a set of graphs illustrative of the ¦operation of the embodiments o~ the inventlon ~hown in Figures 1 ¦and 2, Description of the Preferred Embodiments I __ ¦ The cascaded modelocked laser system in accordance with ¦the sub~ect invention comprises a means to reallze an advancement ¦in laser technology wherein the advantages of both actlve and ¦passive modelocking is utillzed while eliminating thelr limltatlon .

~78702 Referring now to the flgures whereln like reference ~numerals refer to like components, in Figure 1, rererence numeral ¦10 denotes a first modelocked laser having a cav1ty length L
¦defined by the distance between a totally reflecting end ¦mirror 12 and a partlally reflecting output end mirror 14.
¦Intermediate the end mirrors 12 and 14 ls located a laser medium 16 and a modulator crystal 18. As is well known the modelocked laser 19 generates a traln of actively modelocked laser pulses 20 whlch are output from the end mirror 14 and thereafter reflected from the reflective surface 22 whereupon they are directed to a saturable absorber cell 24 which is located within the cavity substantially along the beam axis 25 of a second laser 26 having a cavity length L2, which length i8 an integer multiple of the cavity length L1 Or the first laser 10, i.e, L2 = n Ll, where n is an lnteger.
The second laser 26 is in tandem, i.e., a series arrangement, with the first laser 10 and includes, in addition to the saturable absorption cell 24, a laser medium 28 located bet~een a totally absorbi~g end mirror 30 and a partially reflectivq output end mirror 32. The laser pulses 20 of the first modelocked laser 10 are of sufficient intensity to operate to modulate the loss of the saturable absorber cell 24 of the second mode-locked laser 26 to produce a traln Or modelocked ou'put pulses ~
34. The first laser 10 serves only to modulate the loss of the I
saturable absorber 24 and the modelocking of interest accordingly occurs in the second or output laser 26, The respective cavity lengths Ll and ~2 are selected in accordance with the above-mentioned L2 = n L1 relationship to ensure the synchronism of the respective pulse trains generated by the two lasers 10 and 26 What is significant, howeverJ is that during the onset regime Or the buildup of modelocking in the second laser 26, wherein the initlal laser pulse ls rormed, it behaves as an actively modelocke~

117~7~2 ,lds~r due to the modulation of the saturable absorber by the Ipulses generated by Ll while in its subsequent steady state regime ! it behaves as a passively modelocked laser passing through the ! now saturated absorber as will be explained.
.¦¦ In the second embodiment o~ the sub~ect invention ¦Iwhich ls shown in Figure 2, a cross cavity conflguration i5 dis-closed utllizing the lasers 10' and 26' but whose cavltles as generally deslgnated by the cavity lengths Ll and L2, intersect such that the saturable absorption cell 24 ls common to both ~o cavities as well as intercepting both beam axes 15 and 25 at substantially the same point, The laser 10' of Figure 2 dlffers from the laser 10 of ~igure 1 by the utilization of a totally reflecting end mirror 38 in place of the partially reflecting end mirror 14 and by locatlng the modulator crystal 18 between the laser medium 16 and the end mirror 38. The output laser 26' i8 essentially the same configuration as shown in Figure 1 wlth the exception of` the incluslon of another absorption cell 40 in the second laser cavity along the beam axi~ 25 ad~acent the end mirror 30, In both configuratlons, the laser pulses 20 generated '0 by the active modelocked laser 10 or 10' are directed to the saturable absorptlon cell 24, In the cross cavlty configuratlon Or Flgure 2, advantage ls taken of the higher fields available inside the cavity o~ laser system 10' to modulate the æaturable absorber 24 common to both lasers, Referring now to Figures 3 and 4, Figure 3 shows the transient variation of gain and loss during the bulld up of mode-locking i.n a conventional passively modelocked laser whlle Figure 4 shows the transient variation of galn and loss during the build up of rnodelocking in the second laser system 26 and 26'.
~0 As shown in both figures, the transient gain rises very sharply during the onset regime until the initial pulse 331 and 341 ls formed and thereafter gradually decaying as subsequent pulses 332, 333 .,. and 342~ 343 ,,, increase in amplitude, ll 117~7QZ

With respect to the set of curves in Figure 4, during the onset regime the lnitial pulse 341 f the second laser 26 . or 26' is formed by the modulation imposed on the absorption cell 24; however, during the quasl-steady state regime, pulse shaping is effected by the saturation of the absorptlon cell 24.
As a consequence, during the onset regime, the second laser 26 or 26' behaves as an actively modeloc~ed laser whose lntracavity modulation is provided by the loss modulation ln-duced in the saturable absorption cell 24 by the pulses 20 inci-dent from the laser 10. The modulation, moreover, by selectionof the cavity length Ll and L2 is tuned to the mode spacing in the output laser, or in other words, the gating Or the saturable absorption cell 24 i8 synchronized with the round trip transit time of the pulses 34 in the second laser 26 or 26'. Further-more, the absorption cell 24 is preferably fast acting compared ¦to the pulses 20 from the first laser 10 so that its absorption ¦follow~ instantaneously the train of pulses from laser system ¦10 Thus the saturable absorption cell 24 acts as a ~ast periodic ¦gate within the cavity of the second laser 26 or 26'. Such ¦modulation ls rlcher in higher harmonics than sinusoidal ¦modulation so that it provides coupling over a wider range Or ¦ the cavity mode spectrum Consequently, pulse build up and narrowing is more rapid than in the case of the conventional act of modelocklng Because the second laser 26 or 26' acts as an actively modelocked laser during onset, the stability problem associated with the build up of passive modelocking from noise is circum-l vented As the pulses 341 34n of output pulse train grow I in inten~ity, the saturation of the absorptlon cell 24 becomes ¦ increasingly important. When the ith pulse 34i occurs at the r~gime transition, at which time the absorption cell 24 becomes saturated ~he pulse in the second cavity itself plays a dominant role in th~ shaping of the pulse and as a result in the quasi-steady state regime, and the laser system 26 or 26' behaves as a passivley modelocked laser.
The application of greatest lnterest for the sub~ect ~cascaded 8ystem ls one in which the second laser 26',for example, .
l~s shown in Flgure 2 ls a high power Q or gain switch laser ¦whi.le the ~i.rst laser 10' is a comparatively lower power laser In such an arrangement the galn in the two laser cavltles is widely disparate and accordingly two co-linear absorbers 24 and 40 are required along the beam axis 25 for the system to operate errectively. The absorption cell 24 located at the intersection of the beam aY.es 15 and 25, moreover, is adapted to have a relatively low saturation intensity and a relatively fast relaxation time compared to the round trip transit time rOr the pulses 20 out of laser 10 in order to permit modulatlon of the saturable absorber cell 24. ~he absorption cell 24, because lt is common to the cavity of laser 10', must also have a slgnal absorption characteristic sufrlciently low that laser 10' can lase Because the steady state power levels reached ln the second laser 26' are ~uch greater than in the laser system 10', the additional absorption cell 40 is included to provide absor~er means which exhiblts a comparatlvely high saturation intensity to prevent total bleaching and a relaY~ation time that is fast compared to the modulating pulses 20 to provide pu].se shortening. Thus the saturation absorption cell 24 operate~ to shape the pulses 34 in the second laser ~ at low power levels, i e. durlng the onset ¦
l Or lasing; however, when it saturates as ener~y builds up in the second laser cavlty the absorptlon cell 40 continues to shape t~e pulses 34 once the h~gh power levels of the quasl-steady state re~ime have been achieved.

i~7~70Z

In a ~ystem where the second or output laser comprlses a high power C02 or Nd:glass laser, typical characteristics for the saturable absorber cell 24 for operation at 10 6 micron~ ~r at 1. o6 microns are shown in Table I.

l Table I
¦_turable Absorber Characteristics ¦ Laser - 26' Saturable Operating Pulse Effective l Absorber -24 Freq. Width Is I
¦ C2 SF6 - He 10 6 m1crons 1 sec~9 50 kW/cm2 I
l p-type Ge 10.6 microns 1 sec~l2 10 MW/cm2 I
I Nd:Glass Eastman 1 06 micron~ 10 ~eC-12 1 MW/cm2 ¦ 9740 Dye I
¦ To achieve a 10% modulation of the saturable loss of ¦the absorbers, the peak instant intensity Ip must be approxi-¦mately one-tenth of the absorber saturation intensity I~. Thls ¦sets a lower limit on the peak intensity of the pulses from the actively modelocked laser 10' required to make the cascaded combination operable. For a C02 output laser 26', the pulse intensity required is either 5kW/cm2 or IMW/cm2 depending on whether SF6 or p type Ge is used as the absorber material for the cell 24. Peak intensities of over IMW/cm2 are readily avail-able from actively modelocked TEA C02 lasers operated at atmos-pheric pressure In such a configuration, however, the cascaded system would con~ist of two gain switched C02 lasers triggered such that the pulses from the first C02 laser are incident on the absorber cell 24 during the gain build up of the second C02 laser. In this manner, an actively modelocked subatomic T~A C02 laser can initiate the modelocking of a mult~atmospheric TEA
3o C2 laser.

1~7~70Z

The pulse intensity required for a Nd:glass actively modelocked la~er is approximately lOOkW/cm2. Pulse intensities . Or this magnitude can be obtained from a combination of a Q switched actively modelocked Nd:YAG laser formlng the laser 10' and a Nd:glass laser formlng the laser 26'. The Q switch of the Nd:YAG laser would be triggered such that the pulses 20 generated thereby are incident on the saturable absorption cell 24 during the gain build up of the Nd:glass laser 26'.
Thus the passive modelocking of the Nd:glass laser would be obtained, Thus what has been shown ls a dual modelocked laser system where actively modelocked pulses from one laser are used to modulate the loss of a saturable absorber po~itioned in the cavity o~ a second laser with the cavity lengths being ad~usted so that the modulation is synchronized with the respective pulse transit times.
While the invention has been shown and descrlbed with reference to what is at present considered to be the prererred embodiments thereof, it should be understood that the foregolng has been made by way o~ illustration and not of limitation and accordingly all alterations, changes and modifications coming within the spirit and scope of the inventlon as defined in the sub~oined claims are herein meant to be included,

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A cascaded modelocked laser system comprising in combination:
first modelocked laser means in a first laser cavity including a modulator crystal for generating a first train of actively modelocked laser pulses;
second modelocked laser means, in a second laser cavity having a beam axis and including a saturable absorber located substantially along said beam axis of the second laser means, for generating a second train of modelocked laser pulses;
directing means for directing said first train of modelocked pulses to said saturable absorber which is modulated thereby and wherein, during pulse buildup of said second train of modelocked pulses, said second modelocked laser means exhibits the characteristics of an actively modelocked laser during the onset, build up regime during which the initial pulse therein is formed by the modulation of said saturable absorber and there-after during a steady state regime exhibits the characteristics of a passively modelocked laser, the latter characteristics being due to the saturation of said saturable absorber by said second train of modelocked pulses generated by the second laser means itself; and wherein the cavity length of said second laser means is a multiple of the cavity length of said first laser means thereby ensuring the synchronism of said first and second mode-locked pulse trains respectively generated in said first and second laser means.

2. The system as defined by claim 1 wherein said first and second laser cavities are intercoupled.

3. The system as defined by claim 2 wherein said laser cavities are coupled in series and wherein said saturable absorber means is located in the cavity of said second laser means.

4. The system as defined by claim 2 wherein said directing means comprises an optical reflector means.

5. The system as defined by claim 2 wherein said laser cavities are cross-coupled whereby said saturable absorber is common to both cavities and wherein said first and second trains of laser pulses entering said saturable absorber intersect therein.

6. The system as defined by claim 5 wherein said first laser means comprises a relatively low power laser and said second laser means comprises a relatively high power output laser.

7. The system as defined by claim 3 wherein said first laser means comprises a relatively low power laser and said second laser means comprises a relatively high power output laser.