Self-aligned non-dispersive external cavity

tunable laser

Christoph e Mose r,

1,*

Lawrence Ho,

and Frank Havermeyer

1,2,3

Ondax, Inc., 850 E. Duarte Road, Monrovia, CA, 91016, USA

Corresponding author: moser@ondax.com

Abstract: We are reporting on a novel self-aligned non-dispersive external

cavit y laser (ECL) based on thic k volume hologra phic grat ings (VHG) . The

ECL is tunable and operates with single mode and broad area multimode

laser diodes. We experimentally demonstrate tunable single frequency

operation at 405 nm and 785 nm. The tunable ECL concept is also

experimentally tested with high power broad area laser diodes near 780 nm.

The passive alignment feature of the cavity is expected to reduce the

assembly cost of tunable ECLs.

OCIS codes: (140;2020) Diode lasers; (090.4220) Multiplex holography

References and links

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1. Introduction

State-of-the-art Littrow and Littman external cavity lasers (ECL) use an angularly dispersive

surface diffraction grating as the frequency selective element and a rotation scheme to provide

wavelength tuning [1,2,3]. Although miniaturization of ECLs have been attempted

commercially at the 1.5 μm telecommunication wavelength using MEMs to rotate the

components [4], footprint reduction has been difficult to realize at shorter wavelength. The

#101190 - $15.00 USD Received 5 Sep 2008; revised 1 Oct 2008; accepted 2 Oct 2008; published 3 Oct 2008

main reason is that shorter wavelength requires higher spectral resolution and this tends to

increa se the size o f the optic s.

In contrast to the Littman and Littrow cavities, fixed wavelength ECLs using non-

disp e rsive re f le ctio n volume ho lo gra ph ic gr a tings ( VHG) h a ve be e n re por te d f or t he f irst time

to the authors’ knowledge in 1985 [5]. Since then, others have made fixed wavelength ECLs

using VHGs for single mode and multimode high power lasers [6-9]. Although passive

alignment of an ECL with VHGs has been demonstr ated [8], such an ECL is tu nable only by

controlling the temperature of the VHG, which limits the tuning range to a few tenths of a

nanometer. The current study demonstrates both passive alignment and large tuning range

using VHGs.

2. Comparison of the spectral resolution of ECL with dispersive and non-dispersive

gratings.

In the Littrow cavity (Fig.1(a))the diffraction grating, typically blazed, retro-reflects the

diffraction order back in the direction approximately opposite to the incoming collimated

beam. The zero order of the diffraction is the output of the cavity. Schemes to stabilize the

alignment sensitivity have been devised by introducing optical elements such as cylindrical

lenses between the collimated lens and the diffraction grating [2]. A disadvantage of the

Littrow cavity is that angular tuning of the diffraction grating also changes the direction of the

output beam. The spectral resolution of the Littrow laser cavity is given by [10].

βπ

tan

Littrow

D⋅

=Δ

, (1)

where

is the wavelength of the laser,

D is the diameter of the collimated beam at the

waist and

is the angle formed by the normal of the dispersive grating and the direction of

the co llimated beam. As can be noted fro m equatio n 1, for ma ximum spe ctral re solution, the

diameter of the beam should be as large as possible and at a grazing angle on the dispersive

grating. A larger beam diameter increases the cavity length since either a longer focal length

or intracavity prism expanders [3] are required.

VHG

Laser Diode

D ispersive grating

Laser Diode

(a)

(b)

VHG

Laser Diode

VHG

Laser Diode

D ispersive grating

Laser Diode

(a)

(b)

Fig. 1. (a) Littrow ECL with dispersive grati ng (b) EC L based on non- dispers i ve VHGs.

Figure 1(b) shows an ECL based on a non-dispersive VHG for the spectrally

selective component in the cavity [5]. The VHG acts as a narrowband reflective output

#101190 - $15.00 USD Received 5 Sep 2008; revised 1 Oct 2008; accepted 2 Oct 2008; published 3 Oct 2008