Sharp Actius PC-GP20 Uživatelský manuál stáhnout pdf (Strana 13)

(303) 464-1919 www.dbmoptics.com

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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 dBm Optics

and New Focus the leading supplier of tunable lasers for test and

measurement. All of our 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 in a Laser with

Truly Continuous, Mode-Hop-Free Tuning

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 nA rms current noise in a 1-MHz bandwidth.

Laser Cavity Designs for 24/7 Reliability

Adding our extensive experience in manufacturing lasers and

opto-mechanics 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 15–35 ºC. This means that they can

survive the long-term, 24/7 use (and abuse) found on many

manufacturing floors.

Model 650 Tunable Laser Source

The Technology Behind the Performance

Patent 5319668 issued and patents pending

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