Page 423 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 383
7. CAVITY COMPONENTS

7.1 Coupling Optics
7. I. I Collimating lenses
A number of different types of lenses have been used to collimate the
active-area emission in ECLs. Brief descriptions of the most common types and
their properties are described in this section.

7.1.1.1 Microscope Objectives
These multiple-element spherical lens systems are available with numerical
apertures as high as 0.8. To minimize loss and spurious etalon effects, all external
and internal surfaces should be AR coated. Multiple-element collimating objec-
tives specifically designed for laser diodes [52] are commercially available from
vendors such as Melles Griot and Newport. Care should be used when selecting
collimating objectives since many are designed to be used with a cover glass over
the laser diode. Disregard for this fact will cause additional wavefront distortion.
7.1.1.2 GRIN Rod Lenses
Rod lenses with a radially graded index of refraction are manufactured by
Nippon Sheet Glass and marketed under the name SELFOC [53]. These lenses
are quite useful for ECLs, but they have higher wavefront distortion than the best
multiple-element systems, which probably reduces somewhat the maximum
external feedback that can be obtained. The plano-plano versions have numeri-
cal apertures up to -0.45. A planoxonvex version has an NA of 0.60.
7.1.1.3 Silicon Lenses
Singlet silicon lenses have lower spherical aberration for a given NA
because of the high refractive index of silicon [53]. Because silicon is strongly
absorbing for h < 1.1~". these lenses are only useful for ECLs operating in the
1.3- to 1.5-pm tuning bands. Material dispersion may cause significant chro-
matic aberration and limit the tuning range that can be achieved without working
distance adjustment to less than the full gain bandwidth.
7.1.1.4 Aspheric Lenses
Molded glass and plastic aspheres can be made with low wavefront distortion
and are available with numerical apertures up to 0.55 [55]. Glass is superior to plas-
tic with respect to birefringence. Special high-index glasses reduce the severity of the
aspheric curve needed to correct for spherical aberration. making the lenses easier to
fabricate consistently. Molded aspherics are single-element lenses, so correction of
chromatic dispersion is not possible. Dispersion in the lens material may limit the
wax elength range that can be covered without working-distance adjustment. An ECL
containing a molded-glass aspheric collimating lens has been reported [56].