Technical Note:
New Focus™ Tunable Lasers

What’s Inside Our Tunable Lasers

The laser cavities in our external-cavity diode laser (ECDL) systems and laser modules are the result of many years of experience in designing and manufacturing tunable lasers. Their demonstrated quality and reliability have helped make New Focus the leading supplier of tunable lasers for test and measurement.

All New Focus lasers start out as commercially available semiconductor-diode lasers. These diodes typically operate with several longitudinal modes lasing simultaneously, leading to low coherence and large linewidths. To ensure high coherence, we anti-reflection (AR) coat the diodes so they act only as gain elements. The diode can then be placed in an external cavity that contains wavelength-selective optics so that only a single mode lases at any given time.

Robust, Proprietary AR Coating for Broad Wavelength Tunability

True single-mode tuning requires that the optical feedback is dominated by the external optics, not by reflections from the diode facet. We use a proprietary AR-coating process to reduce residual diode reflectivities to below 0.001—which guarantees true single-mode operation. This process allows us to use nearly any available single-mode diode laser and achieve low reflectivity over a broad wavelength range. In addition, since the lifetime of an ECDL is commonly limited by that of the AR coating, our proprietary process ensures that our coatings last

Precision Mechanics Result

Once the diode is coated, we place it in an external laser cavity that is a modified Littman-Metcalf configuration. In this cavity, a grazing-incidence diffraction grating and a tuning element provide all the necessary dispersion for single-mode operation. In addition, our cavity design allows mode-hop-free tuning. The wavelength in our modified Littman-Metcalf laser is changed by rotating the tuning element, which changes the diffracted wavelength fed back into the cavity. To prevent mode hopping, the cavity length must be kept at a constant number of wavelengths as the laser tunes. This requires positioning the pivot point around which the tuning element rotates with sub-micron accuracy, enabling us to produce lasers with no mode hops.

Tight Environmental Control for Narrow Linewidth

Once single-mode operation is established by the optics in the external cavity, the linewidth of the laser can be affected by acoustic coupling and cavity-temperature variations, each of which can change the cavity length, and electrical-noise coupling, which causes changes in the index of refraction of the diode and in the piezo length (also affecting the cavity length). Every aspect of our laser design aims to minimize these effects. For example, our laser controllers feature current sources with less than 100-nArms current noise

in a 1-MHz bandwidth

Laser Cavity Designs for 24/7 Reliability

Adding our extensive experience in manufacturing lasers and optomechanics to the proprietary AR coating and unique cavity design results in a robust and rugged laser that can withstand rough handling and a variety of environmental conditions. These lasers surpass international shipping standards for shock and vibration and can operate in environments with up to 80% relative humidity from 15–35o C. This means that they can survive the long-term, 24/7 use (and abuse) found on many manufacturing floors

OPTO
A modified Littman-Metcalf configuration.