Piezoelectrically tuned short-cavity dye-laser design

A. J. Cox and Gary W. Scott A. J. Cox is with University of Redlands, Physics Depart­ment, Redlands, California 92373, and Gary W. Scott is with University of California, Riverside, Chemistry De­partment, Riverside, California 92521. Received 19 December 1983. 0003-6935/84/081135-03$02.00/0. © 1984 Optical Society of America. The short-cavity dye laser (SCDL) has proven to be a con­

venient device for producing picosecond duration pulses throughout the visible spectrum. Pumping sources of the SCDL have included a variety of short-pulse lasers such as the mode-locked Nd:glass laser, the mode-locked Nd:YAG laser, the mode-locked ruby laser, and subnanosecond N2-TEA la­sers.1-7 Most reports of SCDL operation are based on designs with a fixed cavity length typically ranging between 50 and 200 μm. Therefore, these lasers have not been tunable, and they simultaneously generate multiple-axial modes within the 20-30-nm gain bandwidth of the dye used in the SCDL. Recently we reported operation of an SCDL which featured two important improvements in operation: single mode, narrow-bandwidth output, and continuous piezoelectrical tuning of this mode over the entire gain bandwidth of any given dye.8 This Letter reports the complete design of this improved version, a piezoelectrically tuned short-cavity dye laser (PZT-SCDL).

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Fig. 1. Schematic drawing of the PZT-SCDL which shows the pi­ezoelectric translator (PZT), aluminum rings (R1-R6), input and output mirrors (Ml and M2), 0 ring (OR), micrometer heads {MC), ball bearings (BB), spacer rods (SR), and dye flow path (shaded

area).

Single-mode operation of an SCDL was accomplished by using an output mirror with the increased reflectivity at the lasing wavelength of ~90% as opposed to the 60% value pre­viously used.8 This design change allowed the cavity length to be as short as ~4 μm while still maintaining sufficiently high cavity Q values for lasing. This extremely short cavity length in turn resulted in separations of the modes of the etalon formed by the cavity mirrors (free spectral range) that were large enough so that only a single mode could oscillate within the broad gain curve of a given dye. Tuning of this mode was accomplished by use of a cylindrically shaped pi­ezoelectric translator which changed the cavity length and thereby varied the wavelength of the mode.

Figure 1(a) shows a cross-sectional side view of the SCDL and Fig. 1 (b), a view from the output end of the SCDL. The laser cavity/dye cell design is basically the same as was used in our earlier multimode SCDL which was described in detail in Ref. 4. Mirrors Ml and M2 form the optical cavity. The laser is axially pumped through mirror Ml which transmits >85% of the pump light and whose second surface reflects >98% of the dye-laser emission. The first surface of mirror M2 reflects ~90% at the lasing wavelength, while its second surface is AR-coated at that wavelength. Both mirrors are on flat (λ/20) quartz substrates that are 2.54 cm in diameter by 0.95 cm thick.

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The volume between the mirrors, which is enclosed by a surrounding 0 ring, forms the dye cell. The two mirrors are cemented into aluminum rings R4 and i?5. Dye solutions can be circulated through the optical cavity by way of holes, which are drilled into ring R5 equipped with tubing fittings. As described in Ref. 4, the dye solution can flow between the 0 ring and mirror M2 through two beveled regions on opposite edges of the mirror.

The optical cavity length is initially adjusted to a particular value using three micrometers MC (Mitutoyo model 151-122), which are held fixed in ring R6 by set screws. This allows single-mode or multimode lasing as desired. The micrometer heads, which press on three 0.635-cm ball bearings embedded in ring i?5, alter the cavity length by compressing the sealing 0 ring between R4 and R5.

Tuning the dye-laser modes is accomplished by incorpo­rating a hollow cylindrical piezoelectric translator (PZT) (Lansing model 21.838), which slides smoothly inside ring R2. Three screws attach the PZT rigidly to end ring Rl, which is itself secured by screws to ring R 2. The other end of the PZT carries a factory-fitted threaded ring which screws into threads cut in adaptor ring R3. Three screws then secure R3 to the mirror ring R4. Rings R2 and R6 are held at a fixed spacing by three 0.95-cm diam spacer rods SR. In our original mul-timode SCDL,4 ring R4 was bolted directly to R2, which, in that case, did not have the present cylindrical structure to accept the PZT.

Pulses from the pump laser are focused through Ml to a small spot (~0.5 mm in diameter) in the dye solution. The diverging SCDL pulsed emission is recollimated by a lens placed at its focal length beyond the active dye-laser volume. For single-mode operation, the cavity length is adjusted to ~4 μm, and the mirrors are brought into parallelism using the micrometers. With a high repetition rate pump laser (Nd: YAG or N2-TEA) mirror parallelism is easily achieved by observing and optimizing the concentricity of the SCDL emission which consists of a bright ring pattern like that from a passive Fabry-Perot interferometer. Cavity lengths l, which are greater than ~5 μm, can be readily determined by mea­suring the free spectral range Δλ of the multimode SCDL output and using the formula l = λ2/2nΔλ, in which n is the refractive index of the dye solution. Once multimode align­ment and lasing are obtained, it is simple to reduce the cavity length using the micrometers until single-mode operation is achieved.

Wavelength tuning the mode across the dye gain curve is accomplished by applying a dc voltage to the PZT which translates Ml relative to M2. This alters the cavity length and thus changes the wavelength of the lasing mode. The linearity of this tuning method with the PZT voltage was demonstrated in Ref. 8.

The laser is mounted by clamps around rings R2 and R6 on a 16.5- X 16.5- X 0.64-cm aluminum baseplate, which is po­sitioned on a ball and slot kinematic mount so that the laser can be aligned relative to the pump beam as needed. The SCDL can be easily removed from this mount (e.g., for dye changing) and then returned to its position of alignment in the optical system. Disassembly for cleaning or changing mirror sets for dyes operating in different spectral regions requires only a few minutes. The small volume of dye that is required (~5 mliter) is usually held in plastic syringes and injected into the cavity as needed. Dye solutions have also been continuously flowed with a slow syringe pump, but generally a static dye solution with no flow has been found satisfactory.

When pumped at 532 nm by the second harmonic of a mode-locked Nd:YAG laser, the SCDL has produced single-

mode pulses with linewidths of ~0.1 nm (FWHM) at 610 nm using rhodamine 6G dye. These pulses were 7-12 psec in duration with energies of a few nanojoules.5 Experiments in which the SCDL output is amplified to a single-pulse energy of a few millijoules are in progress.

This research was supported by the Research Corp. and the Research Committee of the University of California, Riv­erside. We wish to thank J. D. Adams who helped with ma­chining of the SCDL and Marian Hawkes for typing the manuscript.

References 1. F. P. Schafer, Angew. Chem. Int. Ed. Engl. 9, 9 (1970). 2. B. Fan and T. K. Gustafson, Appl. Phys. Lett. 28, 202 (1976). 3. A. J. Cox, G. W. Scott, and L. D. Talley, Appl. Phys. Lett. 31, 389

(1977). 4. A. J. Cox and G. W. Scott, Appl. Opt. 18, 532 (1979). 5. G. W. Scott, J. H. Clark, M. A. Tolbert, S. P. Webb, A. J. Cox, and

G. Renz, IEEE J. Quantum Electron. QE-19, 544 (1983). 6. G. W. Scott, S. G.-Z. Shen, and A. J. Cox, "Tunable subnanosecond

pulses from short cavity dye laser systems pumped with a nitro-gen-TEA laser," Rev. Sci. Instrum. to be published (1984).

7. P. H. Chiu, S. C. Hsu, S. J. C. Box, and H. S. Kwok, "A cascade pumped picosecond dye laser system," IEEE J. Quantum Electron, submitted,

8. A. J. Cox, C. D. Merritt, and G. W. Scott, Appl. Phys. Lett. 40, 664 (1982).

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