Precision MEMS flexure mount for a Littman tunable external cavity laser

Abstract

A tuning element has been constructed using micro-opto-electro-mechanical systems (MOEMS) technology for Littman configuration external cavity tunable lasers. The device is fabricated by deep reactive ion etching of bonded silicon-on-insulator, using a single layer of patterning to integrate a fixed grating and a mirror that is rotated about a virtual pivot by an electrostatic comb drive. The mirror is mounted on a compound flexure. To ensure mode-hop free tuning, the pivot point should lie at the intersection of the mirror and grating planes. Candidate elastic suspensions are compared, and a combination of a cantilever and a portal spring, is selected. Interferometric measurements of electromechanical performance show that the mirror is rotating about a suitable remote point. Preliminary data are given for a Littman external cavity laser operating with a fixed MOEMS structure.

Optical MEMS for Lightwave Communication

“…In practice, at least one extra degree of freedom is required to initially align the cavity mode and grating filter and to compensate for dispersion in the optical components of the cavity. Academic research on MEMS implementations of Littrow [143], [144] and Littman [145] ECSDLs has focused on the development of accurate one-degree-of-freedom actuators that can provide stable mode-hop-free tuning after initial alignment. An interesting alternative is to use a diffractive element with separate phase and amplitude control to avoid macroscopic motion in the external cavity [146].…”

A Real Pivot Structure for MEMS Tunable Lasers

“…Continuous wavelength tuning is more desirable for many applications such as optical communications, spectroscopic analysis and lab equipment, but the tuning structure is more complicated and requires careful design. Continuously tunable lasers are commonly designed in either Littrow configuration [6], [8], [9] or Littman configuration [5], [10], [15]. The former makes use of a blazed grating as the external reflector as shown in Fig.…”

“…One is hybrid integration that makes use of MEMS only for actuation while keeping conventional bulky components for the other functions such as beam collimation and wavelength selection. The final device would have a large size, and manual assembly has to be used [5], [6], [10]- [12], [17], [18], [31]. The other is single-chip integration that makes full use of micromachining to fabricate on the same silicon wafer as many as possible components (actuators, lenses, gratings, and mirrors, etc.)…”

“…It is especially useful for the Littman configuration, which commonly has the pivot arm (i.e., the line from the pivot to the grating) cutting through the gain chip or the external cavity [5], [10], [15], [19], [20]. However, the position of the virtual pivot is determined by the combination of several parameters, so that it is sensitive to the condition changes such as fabrication error, temperature change, and residual stress [10], [21], [22]. For example, a small deviation of fabricated beam width/height could shift the pivot position significantly and actually make the whole design fail.…”

“…This design was implemented to obtain a grating rotation of 1.6 in a MEMS tunable laser [27]. The one-third relationship was also discussed in [10]. However, the actual pivot position is probably not at one-third beam length as will be discussed later.…”