External Cavity Wavelength Tunable Semiconductor Lasers – a Review
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OPTO-ELECTRONICS REVIEW 16(4), 347–366 DOI: 10.2478/s11772-008-0045-9 External cavity wavelength tunable semiconductor lasers a review INVITED PAPER B. MROZIEWICZ* Institute of Electron Technology, 32/46 Lotników Ave., 02-668 Warsaw, Poland External cavity tunable lasers have been around for many years and now constitute a large group of semiconductor lasers featuring very unique properties. The present review has been restricted to the systems based on the edge emitting diode la- sers set-up in a hybrid configuration. The aim was to make the paper as concise as possible without sacrificing, however, most important details. We start with short description of the fundamentals essential for operation of the external cavity la- sers to set the stage for explanation of their properties and some typical designs. Then, semiconductor optical amplifiers used in the external cavity lasers are highlighted more in detail as well as diffraction gratings and other types of wave- length-selective reflectors used to provide optical feedback in these lasers. This is followed by a survey of designs and prop- erties of various external cavity lasers both with mobile bulk gratings and with fixed wavelength selective mirrors. The paper closes with description of some recent developments in the field to show prospects for further progress directed towards min- iaturization and integration of the external cavity laser components used so far to set-up hybrid systems. Keywords: tunable lasers, external cavity, optical amplifiers, wavelength selective mirrors. 1. Introduction index through changes in temperature or pressure as well as by variation of the injection current. The latter causes, Numerous applications of semiconductor lasers, including however, simultaneous and inseparable shifts in the fre- high resolution spectroscopy and wide bandwidth commu- quency, the output power and for a multimode laser the al- nication network systems, require means to tune their out- location of power among the various modes. Furthermore, put to one or more specific wavelengths with a variety of a solitary diode laser is usually sensitive to optical feed- spectral and temporal characteristics. In particular, narrow back. It was observed long time ago that return beam may spectral line-width, high frequency modulation, and widely induce oscillations that appear in the laser output intensity tunable wavelengths that cover the spectral range from vis- and their frequency varies with the cavity length, laser de- ible to far-infrared are demanded. Radiation sources of sign and operating conditions [4]. high power, near-infrared diffraction limited and single fre- To achieve single frequency operation, a special wave- quency are also required for a myriad of scientific applica- length selection mechanism has to be built into the diode. tions such as materials characterization or laser and ampli- Such mechanism may be either based on exploiting a fier pumping and frequency doubling. Pairs of such diode wavelength selective loss which would be lowest for the lasers with appropriately selected frequency were used to selected wavelength or some kind of dispersers can be used generate terahertz waves [1,2] which have already become to serve that purpose. Distributed feedback (DFB) lasers, an important tool for penetrating solid objects in the quest distributed Bragg reflector (DBR) lasers and their more ad- to improve imaging technology. One of the key current vanced versions like sampled-grating DBRs (SG DBR) [5] trends in optical networking is redirecting of a time – divi- and superstructure grating DBRs (SSG DBR) [6], as well sion multiplexed (TDM) trend towards wavelength – divi- as vertical cavity surface emitting lasers (VCSEL), can rep- sion multiplexed (WDM) systems [3]. To provide an access resent this group of devices. Such lasers can be tuned, alas, to laser sources with the precise emission wavelengths re- over only a limited spectral range by changing temperature quired by the system becomes thus a great challenge in set- or the current of the diode. The tuning range may also have ting up any WDM network. gaps in it. In particular, tuning range of the VCSELs is lim- Unfortunately, in ordinary solitary diode lasers, because ited by built-in wavelength resonant structures and there- of the semiconductor’s inherent broad gain spectrum, usu- fore they will be excluded altogether from considerations ally more than one mode will operate simultaneously re- in this review. The easiest way to obtain a really widely- sulting in multiple output wavelengths and broad spectral -tunable laser is perhaps to make a DFB array [7] but due line-width. Such a laser is amenable to control only by ex- to the increasing losses in the coupler, as a number of DBR ternally induced variation of the energy gap and refractive lasers is increased, there is a trade-off between the tuning range and output power. Other monolithic solutions like a *e-mail: [email protected] grating assisted co-directional coupler with rear sampled Opto-Electron. Rev., 16, no. 4, 2008 B. Mroziewicz 347 External cavity wavelength tunable semiconductor lasers – a review grating reflector (GCSR) have been demonstrated and im- differences in the structure and operation [9]. They have plemented [8] yet they suffer from complicated tuning been known since long time ago at least from the year 1964 mechanism requiring the control of three or more currents. [10]. However, their numerous virtues they now owe, However, the laser system can also be forced to operate mainly developments in the field of diode lasers or strictly in a single longitudinal mode by placing the diode laser in speaking semiconductor optical amplifiers (SOA) are pro- an external resonant cavity comprising a wavelength selec- mising. This is why they still are the subject of interest and tor. Through insertion of the wavelength selective elements research and their properties are continuously being im- in the system, the external cavity configuration permits op- proved at pace with general progress in semiconductor eration in a single mode with a line-width which is consid- technology and optoelectronics. Wavelength tunability is erably less than that of a solitary laser diode. Moreover, se- one of the main important features of these lasers and spe- lection and tunability of the emission wavelength by exter- cial attention will be paid to this particular property. To nal control can be executed without the complications asso- emphasize this intention, lasers will be termed “external ciated with variation of the temperature or pumping level. cavity tunable lasers” (ECTL). At the same time, such system retains the high efficiency which is characteristic of the injection laser while provid- 2. External cavity lasers the basics ing an output mode that can be spatially and spectrally su- perior to that of the solitary diode laser. 2.1. Classification and definitions It is the purpose of this paper to review some topics re- lated to the semiconductor lasers that use a waveguide res- Schematic diagrams of different presently known configu- onant cavity extended beyond the solitary edge emitting la- rations of the ECTLs are given in Fig. 1. An elementary ser. Such lasers are called “external cavity lasers” (ECL) or ECL is displayed in Fig 1(a). It consists of a semiconductor sometimes “extended cavity lasers” depending on subtle optical amplifier (SOA) with the mirror like rear facet, Fig. 1. Schematic diagrams of the ECTLs based on the papers indicated in the cited references. Consecutive drawings display configurations that comprise: (a) elementary ECL consisting of a gain medium (SOA), collimating optics, and two mirrors – one of which formed by a high reflectivity coating (HR) on the SOA’s rear facet, (b) Littrow (after Ref. 11), (c) Littman (after Ref. 12), (d) electro-optic filter (after Refs. 13 and 14), (e) acousto-optic filters (AO) (after Refs. 15 and 16), (f) etalon plus wavelength selective mirror (after Ref. 17), (g) Bragg waveguide (after Refs. 18 and 19), (h) multichannel SOA (after Refs. 20 and 21), and (i) multichannel SOA with phase shift section (PS), and Rowland grating (after Refs. 22 and 23). 348 Opto-Electron. Rev., 16, no. 4, 2008 © 2008 SEP, Warsaw a lens, and a reflector. The output from the SOA is colli- or mated and redirected backwards. Function of the SOA usu- cc DDv ==l . (4b) ally performs a diode laser chip in which the rear and front mml2 2nl facet have been coated with the high and very low reflec- tivity films, respectively. The intra-cavity lens is a critical 2.1.2. External cavity laser element in the cavity design. Among various features, it is required that it should have a large aperture ratio in order to On the assumption that the mirror is nonselective and the collect most of the divergent light emitted from the SOA. photon lifetime is significantly longer due to loss-free The reflector should provide optical feedback to excite propagation over a distance L > nl, the cavity mode spacing lasing in the cavity. is Changing position of the reflector will perturb frequency Dl=- l l + , (5a) and spectrum of the emission. In general, reflectivity of the mE mE mE 1 or reflector must be wavelength dependent to enforce single Dv=+ c[(2 nl L )]. (5b). frequency operation of the system. It may be then termed mE “a wavelength selector”. Schemes of the ECLTs with vari- The effects of external feedback on behaviour of an ous wavelength selectors are shown on consecutive draw- ECL are expected to differ characteristically depending on ings of Fig. 1. In the text, to follow we shall discuss some the distance from the SOA to the reflector and on the re- more important features of the respective designs. flectivity of the SOA front facet. When this distance is This paper refers to numerous papers published since smaller than the output coherence length and the reflec- the time the ECL lasers were invented and ipso facto con- tivity is high, the combined SOA and the external reflector taining a variety of notations.