672 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 3, MAY/JUNE 2007 Volume Holographic Grating Wavelength Stabilized Laser Diodes Gregory J. Steckman, Member, IEEE, Wenhai Liu, Ren´ e Platz, Dominic Schroeder, Christophe Moser, Member, IEEE, and Frank Havermeyer (Invited Paper) Abstract—Volume holographic gratings (VHGs) are the key components for producing laser diodes (LDs) with a temperature- stabilized wavelength and narrowed linewidth. We review the unique characteristics of these gratings that make them useful for this application as well as various alternative approaches of stabilizing LDs and their performance. Index Terms—Laser resonators, laser stability, laser thermal factors, volume holographic gratings (VHGs). I. INTRODUCTION T HE USE of volume holographic gratings (VHGs) to stabilize lasers dates back to the mid-1980s [1] with 1.55-μm-semiconductor lasers. Accuwave Corporation fol- lowed by applying optical feedback to visible and near infrared lasers from 670 to 840 nm [2]. However, at that time, lifetime is- sues with the LiNbO 3 -based material prevented the commercial adoption of this technology. Advances in glass-based materi- als [3] have enabled the recent commercialization and broad- ened the application of this technology. By providing wavelength-selective feedback into a laser diode (LD), a VHG can lock the lasing wavelength to that of the grating. This serves to lower the temperature dependence of the wavelength, narrow the spectrum, reduce the aging-related wavelength changes, and in the case of diode arrays, lock each emitter to the same wavelength, producing a much narrower combined spectrum than that in the unlocked arrays. With the rapid developmental pace of high-power laser diode (HPLD) technology during the last decade, LD wall- plug efficiency now reaches 70% [4] with power levels greater than 100 W/bar. HPLDs are well suited for several applica- tions including solid-state laser pumping, material process- ing, medicine, and instrumentation. The center wavelength of HPLDs and their spectral bandwidth are typically spread over a range of 3–5 nm. A narrower emission spectrum in the range of 0.1–0.5 nm and a smaller wavelength tolerance can be ex- tremely beneficial for some applications (e.g., to increase pump- ing efficiency). Manuscript received November 1, 2006; revised xxxxxx. G. J. Steckman, W. Liu, C. Moser, and F. Havermeyer are with Ondax, Inc., Monrovia, CA 91016 USA (e-mail: steckman@ondax.com; wliu@ondax.com; moser@ondax.com; havermeyer@ondax.com). R. Platz and D. Schroder are with Jenoptik Laserdiode GmbH, Jena 07745, Germany (e-mail: rene.platz@jenoptik.com; dominic.schroeder@jenoptik. com). Digital Object Identifier 10.1109/JSTQE.2007.896060 In Section II, we review the key attributes that make VHGs useful as wavelength-stabilizing components. Section III dis- cusses various locking configurations that have been tested, and presents a new lens-free approach to wavelength locking. II. VHGS A VHG is a 3-D image of the interference pattern between two coherent optical fields [5]. It contains both the intensity and relative phase information from the two recording beams. The 3-D character gives the VHG unique features: high diffraction efficiency, simultaneous spatial and spectral selectivity, and mul- tiplexing capability. Holographic recording can be performed with either thin or thick media. When the material in which the hologram is present is thick, Bragg selectivity occurs [6]. Ko- gelnik has analytically studied thick holograms with a coupled- wave analysis in [7]. In order to achieve high-diffraction ef- ficiency, the material must additionally be low loss, and the grating must have a refractive index pattern, not an absorption pattern. VHGs can have narrow spectral response (below 0.1 nm) and a small spatial acceptance angle (below 0.1 ◦ ). A simple VHG formed by the interference of two collimated beams acts as a spectral filter with diffraction efficiency approaching 100%, narrow bandwidth, and precise central frequency. These fea- tures are important for several spectral filter applications such as laser line rejection, separating and combining beams of dif- ferent wavelengths for spectral analysis, signal detection, power combining, wavelength-selective routing and switching, and im- proving LD emission characteristics by locking and stabilizing the wavelength. In contrast to conventional thin diffraction gratings that angu- larly spread the spectral content of a light beam, thick gratings are not dispersive. Hence, they exhibit filter-like properties with more selective spectral and spatial features than other filter- ing technologies. Only the incident illumination satisfying the Bragg condition is efficiently diffracted (filtered), according to the relation λ =2nΛ cos(θ) (1) where λ is the wavelength, n is the bulk refractive index of the media containing the grating, Λ is the grating spacing (period), and θ is the angle of incidence (in media). Fig. 1 is an example of the spectral selectivity possible with a 1.5-mm-thick grating. Only when the Bragg condition is satisfied, does the diffraction efficiency reach its maximum. If the angle of incidence and the wavelength is changed according to (1), the grating can again 1077-260X/$25.00 © 2007 IEEE