An interferometrically tuned and actively modelocked cw dye laser

Volume 36, number 1 OPTICS COMMUNICATIONS 1 January 1981

AN INTERFEROMETRICALLY TUNED AND ACTIVELY MODELOCKED CW DYE LASER

E.E. MARINERO Max-Planek-Institut far biophysikalische Chemie, Abteilung Laserphysik, Postfach 968, D-3400 G6ttingen, Fed. Rep. Germany

and

J. JASNY Physical Chemistry Institute of the Polish Academy of Sciences, Warsaw, Poland

Received 16 September 1980

An interferometer of the Michelson type is used to tune and to actively modelock an argon ion laser pumped cw dye laser. The interferometer constitutes a resonator mirror with a modulated reflecting coefficient. This modulation corre- sponds to a transient Fourier spectrum containing the whole emission band of the dye. In the center of this spectrum, a mode-locked train of picosecond pulses is generated. These pulses are tunable throughout the tuning range of the employed lasing medium in a simple and convenient way. Due to the inherent characteristic broad band output of the cw dye laser, the modulation envelope extends only for a fraction of the entire lasing period. Means of extending the said envelope to cover the complete laser output are discussed.

1. Introduction

In a previous communication [1 ], the use of an interferometer arrangement to tune and actively modelock a flashlamp-pumped dye laser in a simple and convenient way was reported. Pulses between 15 to 80 ps duration with a bandwidth of 0.1 nm were obtained.

Recently, the same interferometer has been used to modelock a cw dye laser oscillator whose output was amplified with a N2-1aser amplifier system. Broadly tunable picosecond pulses of 2 ps duration with powers up to 50 MW were obtained [2].

This communication decribes the lasing character- istics of the argon ion laser-pumped cw resonator which is tuned and is actively modelocked using the said interferometer.

The laser output consists of lasing periods spaced out by times when no laser action occurs. The lasing period is characterized by a sharp spike in the middle of the pulse which contains the modulation envelope and thus, the modelocked train. Although the lasing period lasts for approximately I ms, the 100% mod-

ulation range extends only for a few/as. Extension of the modulated envelope is achieved by using different types of resonators. Modulation of the complete las- hag period is accomplished by using intracavity dis- persive elements or a mirror-grating tuned resonator configuration.

2. Experimental layout

The schematic arrangement of the resonator cavi- ties employed is shown in figs. la to lc.

The interferometer is of the Michelson type in which the optical path length in the two arms is changed rapidly [3]. The path length change is ef- fected by a quartz block RB which is mounted on the shaft of a motor. The other components of the inter- ferometer are blocks BSB, which are kept apart at a distance of 108 nm by MgF 2 spacers, and the 100% reflectors M 4. The construction of the interferome- ter and its mode of operation is extensively discussed in refs. [1] and [31.

One of the mirrors of the astigmatically compen- sated resonator configurations employed, is substi-

0 030-4018/81/0000-0000/$ 02.50 © North-Holland Publishing Company 69

Volume 36, number 1 OPTICS COMMUNICATIONS 1 January 1981

Ar ÷ LASER ._'= ............ .. ................,.M3 ~

[ -

L - o

~M1

- I

In|erfer°m~ BS~ B

M4 ~ RB a

M&

: ~ " LASER

M1 2 I/~

5

Inter~rometer

..... ~:~: ~:~:;~::~,:::,:~:: ................ _ _ ~i T M M2

M3 iii

M1

Interferorne

M4 ~ T q ] ~ / RB

C MZ,

Fig. 1. Interferometrically tuned and modelocked cw dye laser cavities.

tuted by the interferometer in the manner shown in the figure.

The active medium is a jet stream of rhodamine 6G dissolved in ethylene glycol (3.8 X 10 -3 M) which is pumped by a Coherent Radiation (model 52) argon ion laser via a r = 50 mm dielectric mirror M 3. The three cavities use curved mirrors Mland M 2 of 70 mm radius of curvatu/e. Mirror M 2 serves also as an out- put-coupler of 4% transmissivity. The divergent laser output is recollimated with a f = 70 mm achromat. Mirrors M 5 of the ring laser cavity (b) are fiat 100% reflectors.

The effect of line-narrowing on the modulation of the laser output is investigated by introducing disper- sive elements, such as interference filters IF and etalons E, inside the resonators (a) and (b). The configuration shown in fig. lc, uses a mirror-grating combination as an additive line-narrowing element. The use of this combination in cw dye lasers is described in ref. [4]. Mirror M 6 of the "x-configuration" (c) has a reflec- tivity of 45%. Its rear face is A.R. coated and has a wedge of 2 °. The diffraction grating is a 52 mm X 22 mm holographic grating (from Zeiss) with 2442.8 grooves/mm, blazed at 580 nm.

The cavity lengths employed are: cavities (a) and (c): 108 cm, ring resonator = 240 cm.

The pumping power threshold values for the three configurations are: a = 0.6 W, b = c = 2 W. In our ex- periments a pump power level 50% above threshold is used except for investigations concerning the laser behaviour near threshold.

The tuning ranges of the linear and the ring reso- nator (pumped 50% above threshold and without dis- persive elements)are:linear = 565 nm to 615 nm, ring = 571 nm to 605 nm. The corresponding change in the rotational period of the quartz block is: 14.4 ms to 15.32 ms for cavity (a) and 19.8 ms to 21.1 ms for configuration (b). To tune resonator (c) over a large range, both the grating position and the inter- ferometer modulation rate must be simultaneously tuned. The observed tuning range for this resonator is 570 nm to 605 nm. Typical power output values at the maximum of the lasing curve are between 5 mW to 10 mW. The spectral properties of the laser are studied using solid-thin etalons and a 3/4 meter spectrometer (Spex 1700-111). The time evolution and characteristics of the laser output profile are in- vestigated using a Tektronix storage oscilloscope (model 7834). Studies of the modelocked train gen- erated within the pulse modulation envelope are done using a Tektronix 7104 1 GHz oseiUoscope.

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Volume 36, number 1 OPTICS COMMUNICATIONS 1 January 1981

3. Linear cw dye laser output

The output of the linear resonator consists of a pulse which lasts some 0.9 + 0.2 ms characterized by a sharp spike in the middle of the profile. The time during which lasing occurs is approximately the time needed by block RB to sweep an angle of +8 ° from its symmetrical position. Beyond this angle, no lasing occurs. The sharp profile in the center of the output is the envelope of modulation and lasts approximately 1 to 3 #s. Within this envelope, short pulses are pro- duced whose shape varies from purely sinusoidal to perfect mode-locked pulses. In fig. 2a, a photograph of the lasing period of the linear resonator is presented. The interferometer is designed in such a way that mod- ulation of the output should occur within an angle of -+8 ° from the zero position (see ref. [1 ] )~. Thus, a mod- ulation envelope lasting at least some 60~)/~s should be expected during the lasing period. As will be later dis- cussed, the short modulation is due to the intrinsic broad-band behaviour of the interferometrically tuned cw dye laser. Short-pulse profiles within the modula- tion envelope are shown in figs. 2b to 2d. The sinus- oidal profile (fig. 2b) corresponds to pulses produced at both ends of the modulation envelope. Towards the centra of the modulated region, the pulses have the shape shown in fig. 2c. Finally, in the vicinity of the zero angular position of the rotating block, the mode- locked train shown in fig. 2d is generated. This train lasts some 560 ns and the pulse separation corresponds to the cavity round trip of the resonator. At 580 nm, the 108 cm long linear resonator operates at 130 Hz (fur period of rotation = 15.00 ms).

At threshold, the modulation in the laser output extends throughout the entire laser pulse. However, as is to be expected in laser systems at threshold, the output exhibited large amplitude and pulse-shape fluctuations. It is desirable therefore, to pump the resonator well above threshold in order to eliminate the aforementioned fluctuations.

4. Spectral characteristics

The tuning range and bandwidth characteristics of the output of the linear resonator (a) are presented in fig. 3. The wavelength markings correspond to the measured value of )~ for the central spike. From the calibrated spectra, a bandwidth of 38.5 A for the en- tire laser pulse was measured. The modulation enve-

Fig. 2. The linear resonator output: a) the complete lasing profile, b) Pulse shape near the end of the modulation enve- lope. c) Pulses generated in the vicinity of the block's zero position, d) Modelocked train produced in the middle of the modulation envelope.

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Volume 36, number 1 OPTICS COMMUNICATIONS 1 January 1981

Fig. 3. ~ linear 0.15 mm

inside etalon @ez text).

lope, on the other hand, is measured to have a band- width of 2.8 A. The spectrum shown in fig. 3b is ob- tained when a 0.15 mm thick - 33% reflectivity solid etalon is introduced inside the resonator. It is appar- ent from the photograph, that at the green end of the spectrum the extra losses introduced by the etalon have a considerable line-narrowiag effect. The laser output at this end showed modulation throughout. At w a v e l e @ longer than 578 nm, lasing occurs in 5 to 6 etalons modes (FSR = 0.77nm), corresponding to a totalbandwidth of ~38 A, With such an extreme- ly broad band, the/nterferometer cannot be expected to modulate the entire truer-output (see Discussion section), Thus, in order to achieve a more extensive period.of modulation of the ~ period, additional line-narrowing elements mUst be used, in conjunction with the interferometer. The fating characteristics of the ring resonator are next discussed.

5. Ring l a ~ with interferometer

Ideally, one would like to obtain complete mod- ulation of the laser output without using additional dispersive elements whichnmke exten~ve tuning com- plicated. The travoli~g wave proper t i~ of a ring res- onator and the additional degree of modulation in-

Fig. 4. Ring lase~ resonator charactefl~ic output, a) Mod- ulation envelope is considerably longer, b) with interference triter inside resonator, c) with 0.15 mm thick intracavity etalon, d) the laser output with both interference triter and etalon.

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Volume 36, number 1 OPTICS COMMUNICATIONS 1 January 1981

troduced by the optical arrangement of cavity (b) are expected to fulfill this requirement.

In fig. 4a the characteristic lasing profile of the ring resonator is shown. In this case, the envelope of modulation extends for approximately 100/as. This implies that the interferometer modulates effectively in an angular range of-+1.1 ° from the zero position of RB. This is, however, still short of the expected angular range of -+8 ° .

When the symmetry of the ring configuration is altered by positioning the interferometer closer to the curved mirrors, the laser output shows unidirec- tional properties. The dependence of the length of the modulation on line-narrowing was then investi- gated. First, an interference filter is introduced inside the ring resonator. The corresponding output profile is shown in fig. 4b. Two 100% modulated regions, at the center and towards the end of the lasLag period, can be seen in this figure. These envelopes have a duration of ~200/as and are separated by a lasing period where no modulation occurs. When a thin etalon is employed to narrow the lasing bandwidth, envelopes of modula- tion centered at the transmission maxima of the etalon orders are observed throughout the lasLag period. This can be seen in fig. 4c, which shows the characteristic output when a 015 mm thick, 33% reflectivity-solid etalon is used with the ring resonator. To achieve fur- ther line-narrowing, both the interference fdter and the thin etalon are used in combination as dispersive elements. As a result, the modulation envelope ex- tends now practically across the entire 0.9 ms long laser pulse, as shown in fig. 4d.

6. Mirror-grating tuned cw dye laser with interferometer meter

Although nearly complete modulation of the out- put is achieved using two intracavity dispersive ele- ments in the ring resonator, to achieve extensive tun- ability, it is necessary to simultaneously tune 3 ele- ments. A more convenient scheme is to use a diffrac- tion grating as a dispersive element in conjunction with the interferometer. In this case, only two ele- ments need to be adjusted to achieve broad tunahility. In our experiment, we found the configuration shown in fig. lc, a mirror-grating combination, less sensitive to cavity alignment and enabled us to use lower pump-

Fig. 5. a) Modulation in the output of the mirror-grating tuned resonator with interferometer, b) The 100% modulated output of the argon ion laser with interferometer (see text).

Lag powers. The typical output of this cavity is pre- sented in fig. 5a. The modulation envelope stretches across the entire lasing period.

The dependence of the degree of modulation of the laser output on its monochromaticity was further in- vestigated by using the interferometer in:conjunction with the argon ion laser. This laser ~has a linewidth (single line operation) of ~1 GHz find thus, its output when driven interferomettieally,'iS expected to be 100% modulated throughout. Replacing the rear mir- ror by the interferometer, modelocking of the said gas laser was obtained. Emission at 488 nm was achieved when the interferometer's period of rota- tion was set at 36 ms. The output profile of the laser is shown in fig. 5b. Modulation across the complete lasLag period can be observed. If one compares the profiles of figs. 5a and 5b, one can see that the output of the mirror-grating tuned resonator shows an asym- metry which contrasts with the smooth profile of the gas laser. This can be accounted for by the fact that in resonator lc, there are two cavity lengths which can in turn accomodate two oscillation frequencies in the said resonator. It is thus plausible, that lasing takes place simultaneously in two slightly different wavelengths which would in turn result in an asym- metric lasLag profile.

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7. Discussion

a) The output of the interferometrtcflly drive cw dye laser lum:an extremely broad bandwidth which is characterized by a modulation envelope, in the center of the lasingproflie, which extends only for a very short periodof the expected modulation range of the interferometer.

b) Extension of the.modalation envelope demands either operation near threshold Or the introduction of intracavity dispelsive elements.

The short dttration of the modulation envelope and the h r o a d ~ r l u e r o u t p u t are inter-related phe- nomena, the broad band operatio~ me the threshold conditions of the resonator with coupled interfero- meter, the lasing medium is quite energetically pumped. Thus, only a small optical feedback is re- quired to result in lasing.

As the rotating block approaches its operational angular range, the combined reflectivities of the block's surfaces and mirrors M4, are sufficient to reach the threshold requirement for the resonator. This part of the lasing curve is unmodulated and the motion of the block, acting as a rotating mirror, through transient tuning, leads to a broad spectral bandwidth. The interferometer produces a temporal Fourier spectrum within its modulation range, leading to the characteristic central spike. Considering the ap- proximate interferometric relationship A~/X which gives the reciprocal of the number of expected pulses in a Fourier spectrum, for a AX = 38 A, one cannot expect more than ~160 pulses within the modulation range. From this relation it becomes evident that the narrower the lasing bandwidth, the more extensive the modulation envelope ought to be, as conf'Lrmed in our experiments, Thus, in the case of the grating- tuned dye laser which has a bandwidth of ~0.1 A, at

600 nm approximately 60 000 pulses should be ex- pected. With a cavity round trip of 7.2 ns, the mod- ulation envelope is expected to extend at least for ~450 #s. As seen in fig. 5a, the complete ~800 Us long pulse is modulated, the modelocked train, how- ever, extends only for ~300/as. The rest of the enve- lope is purely sinusoidal modulation.

8. Conclusion

Active modelocldng and versatile broad tunability are achieved when a cw dye laser resonator is used in conjunction with the interferometer. The modulation envelope in the output profile extends for a consider- able shorter range than expected.This is due to the inherent broad operation of the dye laser. Complete modulation of the output and a correspondingly larger modelocked train is attained by actively spec- trally narrowing the laser emission.

Acknowledgements

Many fruitful discussions with Prof. F.P. Schiifer are gratefully acknowledged. Our thanks are also ex- tented to W. Sauermarm and D. Ouw for their invalu- able technical assistance.

References

[1 ] J. Jasny, J. Jethwa and F.P. Sch~ifer, Optics Comm. 27 • (1978) 426.

[2] E.E. Marinero and F.P. Schi/fer, Appl. Phys., to be published.

[3] J. Jasny, Polish Patent AppL No. P-204729 from 17. Febr 1978

[4] E.E. Marinero, A.M. Angus and M.J. Colles, Optics Comm. 14 (1975) 226.

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