Frequency tunability and temporal characteristics of a DAMC distributed feedback dye laser
P K PALANISAMY, A R A M A L I N G A M , V M A S I L A M A N I and B M SIVARAM*
Department of Physics, Anna University, Madras 600025, India
*Department of Physics, Indian Institute, of Technology, Madras 600036, India MS received 15 February 1988
Abstract. The frequency tunablity characteristics of a simple prism configuration distributed
feedback dye laser (DFDL) pumped by a low pressure nitrogen gas laser are described.
Tunability is studied as a function of the refractive index of the dye solution and also as a function of the angle of the interfering beams of the pump laser. The tunability range for the dye studied is from 440 to 480nm with a spectral width of 0.1 ~ and the time duration of the DFDL pulses was 50ps.
Keywords. Frequency tunablity; distributed feedback dye laser; Bragg scattering.
PACS No. 42"55
1. Introduction
Dye lasers with distributed feedback are simple and elegant sources of narrow band, frequency tunable, coherent radiation without any complex frequency selective elements. In addition these distributed feedback dye lasers ( D F D L ) are convenient techniques for generating coherent radiation of picosecond duration over the entire visible region. They are therefore useful for several spectroscopic and fast kinetic studies. Here we describe the tunablity characteristics of a prism configuration D F D L system tunable throughout the gain bandwidth of the dye 7 diethyl amino 4 methyl coumarin (7DAMC) lasing medium and capable of giving pulses of 50 ps duration. A further advantage is that both the D F D L and the pumping nitrogen laser can be easily built in laboratories with modest financial resources and workshop facilities since the D F D L system does not contain any sophisticated optical elements.
The operation of a distributed feedback structure was demonstrated by Kogelink and Shank (1971). If the active medium is incorporated into a spatially periodic structure and excited, the counter running waves travelling in the periodic structure receive light along their path by Bragg scattering. This creates a feedback mechanism distributed throughout the length of the active medium. If the feedback is sufficient, oscillation occurs. The spectral selection occurs due to the wavelength selectivity of the Bragg scattering (Kogelink and Shank 1972). In this paper we describe a simple prism-based optical arrangement which makes it possible to obtain narrow linewidth frequency tunable operation of a D F D L .
543
544 P K Palanisamy et al 2. Experimental arrangement
Figure 1 shows the distributed feedback prism-dye cell configuration used for the creation of the interference pattern on the surface of the dye cell (Chandra et al 1972).
The D F D L was pumped by a super-radiant ultraviolet nitrogen laser, built by us using a rear reflector kept at 25 cm to enhance brightness. The distributed feedback for the dye laser was obtained using an isosceles right angled quartz prism. The nitrogen laser pump radiation was condensed by a cylindrical quartz lens into a line image which was incident on the hypotenuse AB of the prism. The light transmitted by the hypotenuse is totally reflected from the side AC of the prism and interferes and forms fringes on a dye cell attached to the prism producing periodic modulation of the refractive index and also of the gain. The feedback is obtained from the Bragg reflection from the periodic structure incorporated throughout the active medium. When pumped by the light of wavelength )~p incident at an angle 0 on the medium, the DFDL wavelength is given by (Shank et al 1971)
/~DFDL = nd2p/np sin 0. (1)
Here nd and np are the refractive indices of the dye solution and the prism material respectively.
3. Results
The above relation (1) shows that the lasing wavelength is a function of the four parameters %, ).p, np and 0 and hence the output can be tuned by varying them. In the present study, the DFDL output was first tuned by varying the refractive index of a DAMC solution in a mixture of methanol and dimethyl suiphoxide by altering the
Y
DFDL oul'puf
B/
s E
n d
A B C - F u s e d q u o r f z prism C B E D - D y e c e l l
Figure i. Schematic diagram of the prism-dye cell.
Figure 2. Spectrographic record of the tuning of DFDL output on changing the refractive index with Hg spectrum as reference.
546 P K Palanisamy et al
proportion of the two solvents. Figure 2 shows the spectrographic record of the tuning of the DFDL output on changing the refractive index with Hg spectrum as reference.
The amplified spontaneous emission from the dye laser was also recorded and superposed over the DFDL output to illustrate the degree of line narrowing. The tuning was linear and the sensitivity observed d2/dnd was 325 nm per unit change in refractive index (see figure 3).
The tuning of the DFDL (DAMC in methoxy ethanol) changing the interfering angle 0 is shown in the spectrogram in figure 4. The tunability range was from 440 to 480 nm.
The tuning is linear as shown in figure 5 and the calculated sensitivity is 17"5 nm/m tad.
It has been shown by Boret al (1982) that a DFDL pumped with a single pulse from a pump laser produces a train of pulses each of several picoseconds time duration. By choosing proper operational parameters (pump pulse width, power and cavity length) it is possible to obtain single pulses of several picoseconds duration from DFDL's (Bor 1980). If the pump intensity does not exceed that for the DFDL threshold by more than 20~o, single pulses of picoseconds duration are generated. In our study the pumping intensity was maintained just above that for the DFDL threshold and the time duration of the single DFDL pulses was indirectly measured using Fabry-Perot etalon.
Measurement of DFDL linewidths gave values ranging from 0"08 A to 0.10A. The maximum linewidth of 0"10A and the assumption that the product of the pulse duration and its bandwidth is 0"6 (Bor 1979) lead us to the conclusion that the DFDL single pulse duration was around 50 ps. This is 150 fold less than the pulse duration of the pump source.
4. Conclusion
We have described a simple arrangement for pumping a DFDL with a nitrogen laser.
The DFDL generates spectrally narrow (0.1 ~), short time duration single pulses ( ~ 50 ps) with the advantage of continuous tuning over several nm (440-480 nm). A
4 8 0
4 6 0
4 4 0
I I
1.36 1-40 t-44
Refractive index Figure 3. ~DEDL a s a f u n c t i o n o f n a.
1"48
Figure 4. Spectrographic record of the tuning of DFDL output on changing the angle of interference 0 with Hg spectrum as reference.
4 8 0
P K Palanisamy et al
E E
. J 4 6 0
a I.L ID
4 4 0
I
- / ,
548
I I I I
- 2 0 2 4
I deg ) Figure 5. "~DFDL as a function of i.
nitrogen laser pumped DFDL of this type will find many applications in high resolution and time domain laser spectroscopy.
References
Bor Zs 1979 Opt. Commun. 29 103
Bor Zs 1980 IEEE J. Quant. Electron. QE-16 517
Bor Zs, Muller A, Racz B and Schafer F P 1982 Appl. Phys. B27 77 Chandra S, Takeuchi N and Hartmann S R 1972 Appl. Phys. Lett. 21 144 Kogelnik H and Shank C V 1971 Appl. Phys. Lett. 18 152
Kogelnik H and Shank C V 1972 J. Appl. Phys. 43 2327
Shank C V, Bjorkholm J E and Kogelnik H 1971 Appl. Phys. Lett. 18 395
