Temperature tunable 1341-nm monolithic Nd3+:YAIO3 laser Donald Scarl

Polytechnic University, Route 110, Farmingdale, New York 11735. Received 28 April 1989. 0003-6935/89/163283-03$02.00/0. © 1989 Optical Society of America.

A short cavity monolithic Nd3+:YAlO3 laser pumped by a forty-stripe diode array has produced 7m W of TEMoo pow­er in only three longitudinal modes. The central mode is smoothly temperature tunable from 1341.0 to 1341.6 nm at the rate of 15.0 ± 0.5 pm/°C.

Diode pumped solid state lasers are rugged and efficient sources of coherent light.1 Short cavity monolithic solid state lasers can provide narrow linewidth, frequency stable, nearly single mode coherent light with lower temperature sensitivity, better mode shape, higher brightness, and higher peak power than presently available diode lasers and with higher efficiency, lower temperature and vibration sensitiv­ity, and smaller size and cost than dye lasers. In addition, solid state lasers are available at wavelengths where neither diode nor dye lasers exist. These miniature lasers are useful for wavelength standards, interferometer calibration, coher­ent fiber optic and space communications, excitation of atomic beams and vapors, and control of higher power lasers by injection.

Solid state lasers will operate in a small number of longitu­dinal modes if these modes are selected by an etalon or waveplate in the cavity,2,3 if standing waves are avoided by using a ring cavity,4 if the laser is gain switched,5 or if the cavity is shortened until only one mode fits under the gain curve.6-8 External cavities containing etalons or birefrin-gent plates require very precise temperature control if the output wavelength is to be kept constant to a few parts per million as required to match a Doppler broadened atomic line or to a few parts per billion to stay within the natural linewidth of atoms in a beam. Ring lasers have been made with monolithic cavities4 but are tunable over a very narrow range, much narrower than the emission linewidth of the crystal material.

Short cavity monolithic lasers (in which the mirrors are formed on the crystal ends) operate in a small number of longitudinal modes when their mode separation is compara­ble with or greater than the crystal emission linewidth. In the case of Nd3+:YA1O3 (YALO) at 1341 nm, a 1.2-mm cavity length leads to 0.39-nm mode spacing, comparable with the ~l-nm gain width of the 1341-nm line. A short cavity laser can be made to operate at any wavelength within the gain curve. The wavelength stability of a monolithic laser de­pends only on changes of the optical path length within the crystal. Since the crystal is small, its temperature and pres­sure can be easily controlled, leading to a wavelength that is stable over long times.

YALO lasers can be tuned to two important He lines: the 2s 3S1-2p 1p manifold centered at 1083.025 nm and the 3p 1P0-5s 1S0 line at 1341.167 nm. A solid state laser locked to one of these lines could form a long-term stable wavelength standard. These lines are also useful for manipulating He atoms in a beam or vapor.9,10

We have used an 807-nm diode array to end pump a short cavity monolithic YALO laser that emitted 7 mW of 1341-nm TEMoo light in three longitudinal modes. The laser wave­length was temperature tunable from 1341.0 to 1341.6 nm.

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Fig. 1. Pump diode spectrum. Fig. 3. YALO laser spectrum. The vertical scale is power per mode rather than spectral power density. The width of the peaks is set by

the monochromator resolution.

Fig. 2. Pump diode angular distribution in the junction plane. The 40-mrad wide bright region was matched into the YALO cavity

mode.

The pump light originated in a forty-stripe 500-mW cw diode array (Spectra Diode SDL2450). Figure 1 shows its fifteen-longitudinal-mode spectrum with a width of ~1.5 nm. Figure 2 shows its 150-mrad wide junction-plane far-field pattern which is ~50 times wider than the pattern expected from a single coherent 400-μm wide emitter. Since the angular pattern perpendicular to the junction plane is approximately diffraction limited, the Strehl ratio for this diode is ~0.02. We discuss below the problem of matching this multimode emitter into a single mode of the YALO cavity.

The diode laser, driven with a current of 1.5 A, was held at a temperature of 5.5°C where its spectrum best matched the YALO absorption line; 450 mW was delivered to the end of the YALO by two spherical lenses and one cylinder lens that converted the 1-μm × 400-μm diode emitting area into a spot ~50 μm in diameter.11

The YALO rod was 1.19 mm long and 3 mm in diameter with its axis along the crystal 6 axis. Its input end, with a radius of curvature of 25 mm, was coated for maximum reflectivity at 1340 nm and antireflection coated at 807 nm. Its output end was flat with a reflectivity of 98% at 1340 nm. The diameter of the almost cylindrical (in the crystal) TEMoo cavity mode was 68 μm. The YALO laser with a threshold of 170 mW and a slope efficiency of 0.035 produced a maximum of 7 mW of TEM00 output power. The far field pattern was circular with an angular diameter equal to that generated by a 1340-nm Gaussian beam with a waist diame­ter of 68 μm.

Fig. 4. Temperature dependence of the wavelength of the YALO laser. The three sets of points correspond to the central mode and

the two side modes.

The expected slope efficiency Es can be estimated as the product of three factors: Ec, the efficiency if all of the pump light can be used by the laser, α, the fraction of the pump light that is absorbed in the length of the crystal, and b, the fraction of the pump light that overlaps the TEM00 cavity mode in the crystal.

For a pump laser with wavelength λp exciting a solid state laser whose output wavelength is λo, whose round-trip loss is L, and whose output mirror transmission is T, the slope efficiency for complete useful absorption of the pump light is

For the YALO laser with an output mirror transmission of T = 0.02 and a round-trip loss of L = 0.008, Ec = 0.43.

The useful fraction a of the pump light that is absorbed in the crystal length Z is

where A is the absorption length for pump light in the crys­tal. With a crystal length of Z = 1.19 mm and an absorption length (averaged over the diode spectrum) of A = 4 mm, a = 0.26.

The fraction b of the pump light that overlaps the cavity mode can be gotten by using paraxial ray matrices on a computer spreadsheet to ray trace diode light into the crys­tal. With the optical system used, junction plane light emit-

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ted within a 40-mrad full width remains within the 68-μm diam 1.19-mm long cavity mode in the crystal. From Fig. 2, the brightest 40-mrad region of the angular distribution contains 34% of the total diode light. Since all the light in the plane perpendicular to the junction remains within the crystal mode, b = 0.34, and the total expected slope efficiency is

in reasonable agreement with the measured value of 0.035. The laser output was polarized with its electric field paral­

lel to the crystal c direction. The crystal a and c directions were determined by measuring the change in focal length when the YALO crystal used as a lens was rotated about its axis. Of the two directions perpendicular to the b axis, the c direction has the lower index of refraction.12

The YALO spectrum as measured by a 0.5-m monochro-mator with a resolution of 0.04 nm and an accuracy of 0.1 nm is shown in Fig. 3. The three longitudinal modes are separat­ed by 0.39 nm, as expected from the cavity length and YALO index of refraction (1.94).

As the YALO temperature is increased, the modes move toward longer wavelengths. Figure 4 shows the wavelength of the central mode and the two sidemodes as a function of temperature. The rate of change of wavelength with tem­perature is 15.0 ± 0.5 pm/°C and is the same for all the lasing modes. The central mode is tunable from 1341.0 to 1341.6 nm by increasing the temperature from 25 to 70°C. Over this temperature interval the total tuning range, including the two sidemodes, is 1340.8-1341.8 nm.

The temperature tuning slope is ~20 times less than that of available 1.3-μm diode lasers, making temperature stabil­ity less critical for the YALO laser than for an equivalent diode laser.

The power in the sidemodes can be reduced relative to that in the central mode by reducing the laser output power. However, in many applications, such as atomic excitation, injection into a higher power laser, or use as a local oscillator, the mode separation of 65 Ghz is sufficient that only one mode will be effective and full output power can be used.

I thank Ralph Burnham, Anu Bowman, Steve Bowman, Barry Feldman, and John McMahon for useful discussions and the loan of equipment. This work was supported by the Laser Physics Branch of the Naval Research Laboratory.

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