External-cavity laser design and wavelength calibration - achieving tuning performance for HP 8167A and 8168A tunable laser sources - Technical

Hewlett-Packard Journal, Feb, 1993 by Emmerich Muller, Wolfgang Reichert, Clemens Ruck, Rolf Steiner

Sophisticated tuning and calibration methods coordinate the effects of a diffraction grating wavelength selector and a Fabry-Perot etalon sidemode suppression filter to ensure accurate wavelength selection and single-mode operation in the HP 8167A and 8168A tunable laser sources.

To achieve the required tuning performance for the HP 8167A and 8168A tunable laser sources, it was decided to develop a tunable laser based on a semiconductor laser chip with an external cavity. There are several ways of building tunable lasers. For example, a three-electrode distributed feedback laser can be tuned over a wavelength range of approximately 10 nm. However, for tuning ranges of 50 nm or greater, lasers with external cavities as resonators and diffraction gratings as wavelength selectors are the solution of choice for most measurements. These lasers are called grating-tuned externalcavity lasers.

External-cavity lasers have been known as tunable sources for a long time and are often used in lab measurements, but have some disadvantages, such as poor power stability, tuning linearity, and wavelength stability. The goal of the tunable laser source development was to develop a stand-alone instrument without these shortcomings of existing externalcavity laser designs.

The two models of HP tunable laser sources are in principle built the same way. Both instruments are external-cavitytuned laser sources. The differences are only in the gain media and some of the optical components. For the HP 8167A the gain is centered at about 1310 nm and the optical components are optimized for a wavelength range of 1250 nm to 1350 nm. In the HP 8168A, the maximum gain of the laser chip is centered at approximately 1540 nm. To cover the wavelength range from 1470 nm to 1565 nm with one instrument, the HP 8168A uses a laser chip that is selected to optimize several parameters. Fig. 1 gives an overview of the architecture and the major components.

Laser Cavity

The gain medium is a conventional laser diode in which the internal laser Fabry-Perot resonator is disabled by an antireflection coating on one laser facet. The coating design and performance are described in the article on page 32. The resonator is then rebuilt by adding an external reflector, the diffraction grating. The grating acts both as a plane mirror and as a wavelength-selective element.

The performance of an external-cavity laser is determined to a large extent by the external cavity. The laser output power, single-mode operation, tuning linearity, and wavelength stability are all are strongly related to the components used in the external resonator. Because of the temperature dependence of these components, the cavity and the laser chip have to be temperature stabilized.

For good performance, the external feedback from the cavity has to be as high as possible. Typical values in the current design are 20% to 30%. This means that the external resonator has roughly the same reflectivity as the former Fabry-Perot cavity. (The laser chip facet reflectivity is a result of Fresnel reflections. The reflectivity calculated from Fresnel's law is approximately 31%.) The collimating lenses shown in Fig. 1 are needed because the laser chip emits a divergent output beam. The beam divergence (full angle at half maximam) of a laser can be as high as 45 degrees for conventional lasers. To capture most of the emitted light the lens has to be designed to handle numerical apertures up to 0.4 and it is required to have a controlled wavefront aberration better than [Lamda/4.]

The cavity design also includes a side-mode filter, which is needed to guarantee a high side-mode suppression ratio. The side-mode filter is a very narrowband wavelength filter that increases the wavelength selectivity compared to just a diffraction grating alone. Its operating concept and how it improves the performance of the external-cavity laser are explained later.

The control and calibration required to tune the sidemode filter and the grating synchronously were a significant challenge. The details are explained later in this article.

Principle of Operation

Fig. 2 shows the interaction between the gain medium, the resonator, the side-mode filter, and the diffraction grating. It also shows how variable wavelength selection is achieved. The grating is tuned by rotation, so the wavelength where the reflection is maximum is dependent on the angle of incidence of the emitted laser beam. The external-cavity laser resonator with its comb-like filter characteristic will allow a large number of possible lasing wavelengths. These possible lasing modes are called "cavity modes." The spacing between two modes is determined by the resonator length and can be calculated. The cavity length is chosen according to the needs of the other optical components and of some specific applications where the cavity mode spacing is important. The mode spacing is about 0.019 nm, corresponding to Af -- 2.5 GHz at a center wavelength of 1540 nm.

The diffraction grating filter bandwidth is determined by the optical beam diameter, the mechanical layout of the filter, and the spacing of the rulings. This is optimized for the external cavity requirements and for manufacturability. In practice, there is a trade-off between wavelength selectivity, reflection efficiency, and manufacturability. A selectivity of about 1 nm FWHM (full width at half maximum) gives good performance with this design.