Laditelný zdroj koherentního záření založený na laserové diodě

Josef Lazar, Ondřej Číp, Petr Jedlička, František Petrů

Semiconductor laser diodes become widespread in the past several years. They are commonly used as transmitters in optical communication systems, CD players, bar code readers and many other commercial applications where high coherence and mode purity are not demanded. There is a lot of laser diodes available on the market from various producers and for acceptable prices working on great variety of wavelengths. The development of LD's leads from types emitting in infrared region of spectra toward shorter wavelengths. During the last several years the red light emitting diodes emerged and shorter wavelength ones will surely follow.

Low price, small size and high output power of LD's led to the effort to use it also in applications where a narrow linewidth, single-frequency regime and high frequency stability is needed. Broad spectral characteristic of the active media promises also broad tuneability and small size and thus low capacity of the semiconductor junction allows high-frequency modulation of the optical frequency and power.

We concentrated our effort on applications putting high demand on the optical frequency stability and coherence - the fundamental metrology. At present the unit of length is defined through the vacuum speed of light and the practical realisation of the length standard is the highly stable laser with its wavelength and thus also its optical frequency of the output radiation varies very little. There are several stabilised laser systems (mostly based on He-Ne laser) accepted by the international comission. They represent only a few points on the scale of optical frequencies. Their stability comparison is very complicated or even impossible.

When we designed our laser system we used the technique of single optical frequency selection developed previously for dye lasers, some time ago the only lasers with a broadband amplifying media. We assembled a laser resonator with the grating as an optical selective element transforming the wavelength to the angle displacement of the beam. When a suitable geometry of the end-mirror movement is selected it is possible to synchronise the tuning of the grating reflection and resonator length the way the laser wavelength is fluent, without mode hops.

The long-term stability of traditional standards is achieved by a feedback regulation of the laser wavelength which is derived from very narrow absorption spectral lines of various gasses. We also applied this technique with some modifications. The absorption medium we used iodine vapour with a dense net of spectral lines in the visible region of spectra. The effort of several metrological laboratories (including our team) was to build a laser system close to the 633 nm wavelength of the common He-Ne laser which serves as a present length standard. Covering of this wavelength by the new semiconductor laser systém allowed also direct stability comparison.

Our team participated on the first international comparison of semiconductor lasers stabilised by means of absorption in iodine vapour under the supervision of the International bureau of weights and measures, BIPM in Paris, France, the metrological institute supervising the primary standards of fundamental physical quantities. The comparison clearly proved the long-term stability of optical frequencies of the semiconductor laser systems to be on the level of the present primary standard and more they allow to cover much broader region of optical spectra by a dense "scale" of stable frequencies.

The experimental setup. APD: avalanche photodetector, F.-P.: scanning Fabry-Perot cavity, L: lens, FC: fiber coupling, M: mirror, RM: removable mirror, PBS: polarizing beamsplitter, l/2: half-wave plate, l/4: quarter-wave plate, FI: Faraday isolator, He-Ne: free-running He-Ne laser, ECL: extended-cavity diode laser, I2: 300 mm long iodine cell, C: Peltier cooler.

Arrangement on the optical table