1836 OPTICS LETTERS / Vol. 21, No. 22 / November 15, 1996

Four-pass amplifier for the pulsed amplification ofa narrow-bandwidth continuous-wave dye laser

Eun Seong Lee and Jae Won Hahn

Division of Quantum Metrology, Korea Research Institute of Standards and Science, P.O. Box 102 Yusung, Taejon, Korea 305-600

Received July 11, 1996

We demonstrate a new type of four-pass dye laser amplifier that can reduce the possibility of parasitic oscillationbetween optical components used in the amplifier. Pumping the amplifier with a 5.6-mJ Q-switched doubledNd:YAG laser output, we obtain high-peak-power pulsed output of an incident cw narrow-bandwidth dye laserbeam with a power gain greater than 2 3 106 . Subsequent amplification of the pulse with a conventionaldye amplifier yields 42% energy efficiency. When a temporally stretched pumping pulse is used, the eventualbandwidth of the final output is measured to be 130 MHz. 1996 Optical Society of America

Since the introduction of pulsed dye lasers, many stud-ies have tried to achieve single-longitudinal-mode op-eration for high-resolution spectroscopic application.1 – 3

Most have used highly dispersive elements such asdiffraction gratings or a Fabry–Perot etalon inside thelaser cavity. In many cases, however, the pulse dura-tion is too short to establish a single longitudinal modeand results in a bunch of modes whose frequencies jit-ter from shot to shot within the overall bandwidth.The other method is the injection-locking process ofa narrow-bandwidth cw dye laser, in which anotherexternal laser cavity is used for the pulse laser os-cillator.4 – 6 The characteristics of the pulse outputcombine the power of the pulsed laser and the spec-tral qualities of the cw laser. However, this methodrequires a complicated control sequence for simultane-ous frequency tuning of the external oscillator with thecw master oscillator. Recently, several authors haveinvestigated an alternative method, that is, the directpulsed amplif ication of a cw narrow-bandwidth dyelaser beam.7 – 9 This method is quite simple in prin-ciple and gives good single-mode stability during fre-quency tuning. For high gain, however, it is essentialfor the cw beam to pass through the active gain mediumof the amplif ier several times. For this purpose, inthis Letter we introduce a four-pass dye amplifier thatis different from previous amplif iers used for otherapplications.

An amplifier in which the laser beam travels backand forth four times on a single line through a gainmedium, changing its direction and polarizationalternately, was first reported by Andreyev andMatveyev.10 They adopted the four-pass configura-tion shown in Fig. 1(a) for the amplif ication of anincident pulse in a solid-state laser system. Later,Han and Kong reported that this configuration couldcompensate for the depolarization caused by thermalbirefringence induced during f lashlight pumping ofa solid-state gain medium.11 The depolarization isknown to result in parasitic oscillation between M1and M2, which gives the output pulse shape a severelymodulated tail. Avoiding this oscillation requirescareful optical alignment so that the beam depolar-ized at GM1 can exactly follow its trajectory after

0146-9592/96/221836-03$10.00/0

ref lecting back at M1 and thus return to its originallinear polarization state after passing through themedium again. However, this is applicable only for asolid-state medium where the depolarization is causedmainly by birefringence.

In cases where dye solution is used as a gainmedium, the depolarization is induced by anisotropicdistribution of gain and loss that occurs with a linearlypolarized pumping laser f ield.12,13 Therefore the po-larization cannot be restored and generally increasesinstead. In this Letter we build a new configurationfor four-pass amplification of a dye laser as shown inFig. 1(b) and investigate its operational characteris-tics through the amplif ication of a cw ring dye laser.To reduce the depolarization degree, the amplif ier cell(GM2) is placed between two crystal polarizers (P4 andP5). We use only one Faraday rotator (FR3) and a pairof Fresnel rhomb polarization rotators (PR2 and PR3).The incident cw beam is propagated through P3, FR3,PR2 (45±), and P4 and reaches dye gain medium GM2.After GM2, the beam passes through P5 and ref lectstwice at M3 and M4, and then its polarization rotates

Fig. 1. Schematic diagram of two kinds of four-passamplif ier: (a) Andreyev and Matveyev,10 (b) this Letter.P1–P5, polarizers; FR1–FR3, Faraday rotators; PR1–PR3,polarization rotators; M1–M6, mirrors; GM1, GM2, gainmedia.

1996 Optical Society of America

November 15, 1996 / Vol. 21, No. 22 / OPTICS LETTERS 1837

90± through PR3. Therefore the beam can be ref lectedto GM2 by P4. After passing through GM2 the sec-ond time, the beam ref lects at P5 to M5 and then backto GM2 again via P5. It then follows the same pathas before but in the reverse direction, with polarizationrestored to the incident state by PR3. Passing throughGM2 a fourth time, the beam is propagated through P4and, after PR2 and FR3, the polarization of the beamrotates by 90±. As a result, the beam leaves the opticalsystem after ref lecting at P3, having passed throughthe dye gain medium four times and been amplif iedat each step. As we can see, in this process the beaminitially depolarized at GM2 becomes polarized by P4or P5, and thus the depolarized state is not incidentupon the gain medium again; consequently the degreeof depolarization does not increase further. But in theconfiguration of Fig. 1(a), because no polarizer can beplaced between GM1 and M1, the beam reenters GM1by M1 with its polarization distorted. So the depolar-ization gets worse, resulting in a greater possibility ofoscillation.

The cw ring dye laser system (Coherent 899), usedas a cw-tunable narrow-bandwidth radiation source,is pumped by a cw Ar-ion laser (Coherent Innova 300)and gives a well-characterized beam of 1-MHzbandwidth. The dye cell is similar to the prism cell de-veloped by Bethune14 and filled with 1 3 1024 M Rho-damine 6G in methanol. It has a bore size of 30 mmlength 3 1.5 mm diameter, and it is transverselypumped with a frequency-doubled Nd:YAG laserwith a repetition rate of 10 Hz (Quantel YG661-10).The pumping laser is operated in injection-seededsingle-mode operation to prevent axial mode-beatingf luctuations of laser pulse and to give a smoothpulse shape. The pulse-shape f luctuation is knownto increase the bandwidth of the dye laser pumpedby it.7,15 The original pulse width of the pumpingbeam is 8 ns, and we split the pulse into two pulsesin the ratio 3:7 and then delayed the stronger pulsetemporally by approximately 5 ns and recombined itwith the smaller one at the dye cell.9,15 The result isa simulated pump laser with a pulse width of 12 ns,improving the bandwidth of the amplif ied outputpulses both by temporal stretching and by lowering thepeak electric field strength present in the cell.

We chop the cw beam before polarizer P3, using afast mechanical shutter with an open interval of 1 ms,to prevent the cw beam from remaining in the dye cellduring most of the time interval when it is not pumped.Finally, the output pulse from the four-pass amplifieris forwarded to the main amplif ier to extract more en-ergy from the active dye medium. The main ampli-fier also consists of the same type of dye cell as thatused in the four-pass amplif ier, but the bore diame-ter is 2.3 mm. The pumping energy ratio between thepreamplifier and the main amplifier is 1:12. To controlthe divergence of the final output beam we place a tele-scope between two amplif iers that consists of a lens–pinhole–lens combination. The pinhole size is 50 mmin diameter and also serves as a spatial f ilter.

From the four-pass amplifier, we obtained a 1.5-mJpulse at 5.6-mJ Nd:YAG pumping energy with 100-mWsingle-frequency cw beam seeded at 573 nm; this cor-

responded to a gain of approximately 2 3 106 and anefficiency of nearly 27%. The pulse energy was mea-sured with a pyroelectric joulemeter (Molectron J4-05);because the energy was too high, we had to attenu-ate the intensity with a neutral-density f ilter (outsidediameter 0.75). The resulting pulse width was mea-sured to be 7 ns. From the result, it is easy to seethat the more passes are possible, the more energycan be extracted from the active medium. However,we found that more than four passes is theoreticallyimpossible in setting up a multipass amplifier. Thisis related to the fact that only four photon states arepossible in one-dimensional space, that is, the combi-nations of two different directions and two polariza-tion states. Therefore, for more than four passes, eachbeam pass must inevitably be shifted relative to theother inside the amplif ier.9 By using a Bethune-typedye cell, one can obtain a good spatial profile of theoutput beam without additional spatial filtering insidethe amplifier. In the configurations of the four-passamplifier shown in Figs. 1(a) and 1(b), one can easilyfind that no additional optical isolation device is neces-sary for blocking the high-peak-power pulse radiationfrom the dye cell, which can cause unlocking or insta-bility in the frequency-stabilization sequence of a cwring dye laser system. Any intense radiation from thedye cell cannot pass through P3 via P4, PR2, and FR3.The isolation ratio of the configuration in Fig. 1(b) wasmeasured to be .30 dB.

To confirm that the cw beam was successfullyamplified, we measured the spectrum of the outputbeam as shown in Fig. 2(a). We used a single-gratingmonochromator (Jobin-Yvon THR 1000) for measure-ment with resolution of ,0.05 nm. In the spectrum,amplified spontaneous emission (ASE) that spread over20 nm from 560 to 572 nm was greatly depleted bypulsed amplification of the cw laser beam, by a factorof 4.5, and the laser beam at 573 nm was greatly en-hanced, whereas the beam was negligible without thepumping beam. We observed a reduction of the ASEportion as the cw intensity was increased. Withoutusing any highly dispersive element or any frequency-selection device, we could reduce the ASE to less than15% of total radiation energy at 573 nm. We also ob-served that the ASE portion could be further reducedas the wavelength of cw laser approached 566 nm,which corresponded to peak intensity of the ASE.

Fig. 2. Spectra of the output from a four-pass amplif ier (a)with and (b) without the cw laser beam.

1838 OPTICS LETTERS / Vol. 21, No. 22 / November 15, 1996

Fig. 3. Bandwidth measurement of the output laser beam.The free-spectral range of the etalon used is 3 GHz, and thefinesse is 125.

The output pulse from the four-pass amplifier wasthen forwarded to a main amplif ier. The dye solu-tion used was the same as that in the four-pass am-plifier and pumped by the same Nd:YAG laser. Wefinally obtained a pulse energy of 32 mJ with 6.5-ns du-ration at total pumping energy 77 mJ. We measuredthe bandwidth of the pulse, using a confocal scanningFabry–Perot interferometer (Burleigh) with a 3-GHzfree-spectral range and a finesse of 125. The result isshown in Fig. 3, in which the bandwidth was measuredto be less than 130 MHz. To see the effects of tem-poral stretching of the pumping laser pulse width onthe bandwidth, we tried pumping the amplif ier with-out stretching. In this case, the measured bandwidthwas 180 MHz.

In conclusion, we have demonstrated a new configu-ration of a four-pass amplifier for pulsed amplificationof a cw narrow-bandwidth dye laser. It is found that

the power gain of the four-pass amplif ier reaches2 3 106. When we use an additional dye amplifier, weobtain an energy eff iciency of 42% and a bandwidth of130 MHz.

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